TextDecoder is slow.

in #39301 we talked about marking string_decoder as legacy but we also brougth up the perf of String_decoder vs TextDecoder, but we didn't talk much about if there was something wrong with TextDecoder and how it performed agains other environments and why it's slower. or if it can be approved upon.

I conducted a test that runs 10k times in different environment and tested to see how fast it is able to do it's job and nodejs didn't do a good job...

here is the results i got

	smallUint8	bigUint8
chrome	78 ms	1259 ms
safari	70 ms	441 ms
Firefox	92 ms	2889 ms
NodeJS	360 ms	4167 ms
Deno	569 ms	1386 ms

(Lower is better)
buffer.toString('utf8') and string_decoder is considerable much faster, maybe you can steel something from it and apply it onto the TextDecoder? or do exactly what safari is doing :)

And the test i run:

const {bigUint8, smallUint8} = fixture()
const decoder = new TextDecoder()

console.time('small TextDe decode')
for (let i = 0; i < 100000; i++) decoder.decode(smallUint8)
console.timeEnd('small TextDe decode')

console.time('big TextDe decode')
for (let i = 0; i < 100000; i++) decoder.decode(bigUint8)
console.timeEnd('big TextDe decode')

function fixture () {
const smallUint8 = new Uint8Array([226, 132, 149, 32, 226, 138, 134, 32, 226, 132, 149, 226, 130, 128, 32, 226, 138, 130, 32, 226, 132, 164, 32, 226, 138, 130, 32, 226, 132, 154, 32, 226, 138, 130, 32, 226, 132, 157, 32, 226, 138, 130, 32, 226, 132, 130, 44, 32, 226, 138, 165, 32, 60, 32, 97, 32, 226, 137, 160, 32, 98, 32, 226, 137, 161, 32, 99, 32, 226, 137, 164, 32, 100, 32, 226, 137, 170, 32, 226, 138, 164, 32, 226, 135, 146, 32, 40, 65, 32, 226, 135, 148, 32, 66, 41, 44])
const bigUint8 = new Uint8Array([60,33,68,79,67,84,89,80,69,32,104,116,109,108,62,10,60,104,116,109,108,32,108,97,110,103,61,34,101,110,34,62,10,10,60,104,101,97,100,62,10,32,32,32,32,60,109,101,116,97,32,99,104,97,114,115,101,116,61,34,85,84,70,45,56,34,62,10,32,32,32,32,60,116,105,116,108,101,62,104,116,116,112,98,105,110,46,111,114,103,60,47,116,105,116,108,101,62,10,32,32,32,32,60,108,105,110,107,32,104,114,101,102,61,34,104,116,116,112,115,58,47,47,102,111,110,116,115,46,103,111,111,103,108,101,97,112,105,115,46,99,111,109,47,99,115,115,63,102,97,109,105,108,121,61,79,112,101,110,43,83,97,110,115,58,52,48,48,44,55,48,48,124,83,111,117,114,99,101,43,67,111,100,101,43,80,114,111,58,51,48,48,44,54,48,48,124,84,105,116,105,108,108,105,117,109,43,87,101,98,58,52,48,48,44,54,48,48,44,55,48,48,34,10,32,32,32,32,32,32,32,32,114,101,108,61,34,115,116,121,108,101,115,104,101,101,116,34,62,10,32,32,32,32,60,108,105,110,107,32,114,101,108,61,34,115,116,121,108,101,115,104,101,101,116,34,32,116,121,112,101,61,34,116,101,120,116,47,99,115,115,34,32,104,114,101,102,61,34,47,102,108,97,115,103,103,101,114,95,115,116,97,116,105,99,47,115,119,97,103,103,101,114,45,117,105,46,99,115,115,34,62,10,32,32,32,32,60,108,105,110,107,32,114,101,108,61,34,105,99,111,110,34,32,116,121,112,101,61,34,105,109,97,103,101,47,112,110,103,34,32,104,114,101,102,61,34,47,115,116,97,116,105,99,47,102,97,118,105,99,111,110,46,105,99,111,34,32,115,105,122,101,115,61,34,54,52,120,54,52,32,51,50,120,51,50,32,49,54,120,49,54,34,32,47,62,10,32,32,32,32,60,115,116,121,108,101,62,10,32,32,32,32,32,32,32,32,104,116,109,108,32,123,10,32,32,32,32,32,32,32,32,32,32,32,32,98,111,120,45,115,105,122,105,110,103,58,32,98,111,114,100,101,114,45,98,111,120,59,10,32,32,32,32,32,32,32,32,32,32,32,32,111,118,101,114,102,108,111,119,58,32,45,109,111,122,45,115,99,114,111,108,108,98,97,114,115,45,118,101,114,116,105,99,97,108,59,10,32,32,32,32,32,32,32,32,32,32,32,32,111,118,101,114,102,108,111,119,45,121,58,32,115,99,114,111,108,108,59,10,32,32,32,32,32,32,32,32,125,10,10,32,32,32,32,32,32,32,32,42,44,10,32,32,32,32,32,32,32,32,42,58,98,101,102,111,114,101,44,10,32,32,32,32,32,32,32,32,42,58,97,102,116,101,114,32,123,10,32,32,32,32,32,32,32,32,32,32,32,32,98,111,120,45,115,105,122,105,110,103,58,32,105,110,104,101,114,105,116,59,10,32,32,32,32,32,32,32,32,125,10,10,32,32,32,32,32,32,32,32,98,111,100,121,32,123,10,32,32,32,32,32,32,32,32,32,32,32,32,109,97,114,103,105,110,58,32,48,59,10,32,32,32,32,32,32,32,32,32,32,32,32,98,97,99,107,103,114,111,117,110,100,58,32,35,102,97,102,97,102,97,59,10,32,32,32,32,32,32,32,32,125,10,32,32,32,32,60,47,115,116,121,108,101,62,10,60,47,104,101,97,100,62,10,10,60,98,111,100,121,62,10,32,32,32,32,60,97,32,104,114,101,102,61,34,104,116,116,112,115,58,47,47,103,105,116,104,117,98,46,99,111,109,47,114,101,113,117,101,115,116,115,47,104,116,116,112,98,105,110,34,32,99,108,97,115,115,61,34,103,105,116,104,117,98,45,99,111,114,110,101,114,34,32,97,114,105,97,45,108,97,98,101,108,61,34,86,105,101,119,32,115,111,117,114,99,101,32,111,110,32,71,105,116,104,117,98,34,62,10,32,32,32,32,32,32,32,32,60,115,118,103,32,119,105,100,116,104,61,34,56,48,34,32,104,101,105,103,104,116,61,34,56,48,34,32,118,105,101,119,66,111,120,61,34,48,32,48,32,50,53,48,32,50,53,48,34,32,115,116,121,108,101,61,34,102,105,108,108,58,35,49,53,49,53,49,51,59,32,99,111,108,111,114,58,35,102,102,102,59,32,112,111,115,105,116,105,111,110,58,32,97,98,115,111,108,117,116,101,59,32,116,111,112,58,32,48,59,32,98,111,114,100,101,114,58,32,48,59,32,114,105,103,104,116,58,32,48,59,34,10,32,32,32,32,32,32,32,32,32,32,32,32,97,114,105,97,45,104,105,100,100,101,110,61,34,116,114,117,101,34,62,10,32,32,32,32,32,32,32,32,32,32,32,32,60,112,97,116,104,32,100,61,34,77,48,44,48,32,76,49,49,53,44,49,49,53,32,76,49,51,48,44,49,49,53,32,76,49,52,50,44,49,52,50,32,76,50,53,48,44,50,53,48,32,76,50,53,48,44,48,32,90,34,62,60,47,112,97,116,104,62,10,32,32,32,32,32,32,32,32,32,32,32,32,60,112,97,116,104,32,100,61,34,77,49,50,56,46,51,44,49,48,57,46,48,32,67,49,49,51,46,56,44,57,57,46,55,32,49,49,57,46,48,44,56,57,46,54,32,49,49,57,46,48,44,56,57,46,54,32,67,49,50,50,46,48,44,56,50,46,55,32,49,50,48,46,53,44,55,56,46,54,32,49,50,48,46,53,44,55,56,46,54,32,67,49,49,57,46,50,44,55,50,46,48,32,49,50,51,46,52,44,55,54,46,51,32,49,50,51,46,52,44,55,54,46,51,32,67,49,50,55,46,51,44,56,48,46,57,32,49,50,53,46,53,44,56,55,46,51,32,49,50,53,46,53,44,56,55,46,51,32,67,49,50,50,46,57,44,57,55,46,54,32,49,51,48,46,54,44,49,48,49,46,57,32,49,51,52,46,52,44,49,48,51,46,50,34,10,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,32,102,105,108,108,61,34,99,117,114,114,101,110,116,67,111,108,111,114,34,32,115,116,121,108,101,61,34,116,114,97,110,115,102,111,114,109,45,111,114,105,103,105,110,58,32,49,51,48,112,120,32,49,48,54,112,120,59,34,32,99,108,97,115,115,61,34,111,99,116,111,45,97,114,109,34,62,60,47,112,97,116,104,62,10,32,32,32,32,32,32,32,32,32,32,32,32,60,112,97,116,104,32,100,61,34,77,49,49,53,46,48,44,49,49,53,46,48,32,67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return {smallUint8, bigUint8}
}

@jasnell

@jasnell

Yeah, it was implemented to be spec compliant and not really to be performant. I'm sure there is lots of room for improvement. I'd love to see someone take on that work.

What I found in the mentioned issue is that the time is mostly spent in this function:

node/src/node_i18n.cc

Line 432 in e2148d7

void ConverterObject::Decode(const FunctionCallbackInfo<Value>& args) {

In my machine, the test code ran in Chrome results in: 49 and 834ms. But Node.js results in 113 and 1730ms.

When I add two lines in ConverterObject just after `ucnv_toUnicode',

args.GetReturnValue().Set(Buffer::New(env, 0).ToLocalChecked());
return;

That means DO NOT do the left logic after decoding via ucnv_toUnicode. It results in 73 and 933ms. I think the left logic is not that fast.

btw, it seems ret = ToBufferEndian(env, &result); is very slow.

All I/O optimizations are multiplied by zero, when decoding text, this main cases for clients - Redis, PostgreSQL, etc...

It seems #42695 fixes this issue.

node

PS F:\node> F:\node\out\release\node --expose_gc 1\1.js                                                                                                                                      small TextDe decode: 209.075ms
big TextDe decode: 1.765s

PS F:\node> node --expose_gc 1\1.js                    
small TextDe decode: 170.5ms
big TextDe decode: 3.058s

chrome

small TextDe decode: 75.279052734375 ms
big TextDe decode: 1370.48095703125 ms

Here's the output before after 2 pull requests I opened on TextDecoder:

Before (v18.11.0):

small TextDe decode: 51.216ms
big TextDe decode: 924.02ms

After (Main):

small TextDe decode: 23.65ms
big TextDe decode: 773.155ms

Bun:

➜  node-test bun ./text-decoder.js
[23.51ms] small TextDe decode
[129.33ms] big TextDe decode

Deno:

➜  node-test deno run text-decoder.js
small TextDe decode: 50ms
big TextDe decode: 439ms

@srl295

@srl295 do you have any suggestions for optimizations on large values?

@srl295

@srl295 ... as a reminder for context... this is making use of the ucnv_toUnicode API. We don't preflight anything, just allocate a large enough space, call ucnv_toUnicode, and process the result into a v8 string.

optimize setLanguage, setScript, shapeWithTrace

FWIW I quickly checked the OP snippet in a node v24.14.1 script:

small TextDe decode: 22.908ms
big TextDe decode: 275.741ms

And pasting in Chrome devtools:

small TextDe decode: 28.212890625 ms
big TextDe decode: 381.914794921875 ms

Which is about the extra const I'd expect for the devtools overhead.

So probably worth re-testing this properly?

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It will be automatically closed in 30 days if no further activity occurs. If this is still relevant, please leave a comment or update it to keep it open.

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TextDecoder is slow. #39879

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TextDecoder is slow. #39879

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ronag commented on Aug 25, 2021

jasnell commented on Aug 25, 2021

targos commented on Aug 25, 2021

XadillaX commented on Aug 26, 2021

XadillaX commented on Aug 26, 2021

uasan commented on Sep 28, 2021

liuxingbaoyu commented on May 7, 2022

anonrig commented on Nov 8, 2022

anonrig commented on Nov 8, 2022

anonrig commented on Nov 8, 2022

jasnell commented on Nov 8, 2022

simonbuchan commented on May 26, 2026

github-actions commented on Aug 25, 2026

github-actions commented on Sep 25, 2026

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