PoW performance

Posted September 1, 2021 by Aravinth Manivannan ‐ 2 min read

Last Edited February 13, 2023

We are mCaptcha. We build kickass CAPTCHA systems that give (DDoS) attackers a run for their money. And we do all of this without tracking your users. Oh and did I mention our UX is great?

mCaptcha uses a proof-of-work(PoW) mechanism to rate limit users or potential bots. In order for this to be effective, the PoW should be configured properly. The difficulty requirement can’t be too high, as it could cause accessibility issues on the client-side while at the same time, it shouldn’t be too low, as it wouldn’t offer proper protection against bots.

Malicious bots(the ones that wreak havoc), run native code which is capable of running in a multi-threaded context. This creates an unfair advantage for crackers using these bots over legitimate users, who usually use browsers to access a website.

I wanted to see how much of an advantage a native program would have over our WASM library.

Benchmark tools

So I wrote these to compare native and WASM performances:


The tests were run on my development machine featuring an Intel Core i7-9750h.


Difficulty Real User Sys
500000 0m0.220s 0m0.197s 0m0.006s
1000000 0m0.203s 0m0.203s 0m0.000s
1500000 0m0.198s 0m0.198s 0m0.000s
2000000 0m0.203s 0m0.203s 0m0.000s
2500000 0m0.758s 0m0.752s 0m0.003s
3000000 0m0.776s 0m0.769s 0m0.003s
3500000 0m2.010s 0m1.998s 0m0.000s
4000000 0m2.038s 0m2.033s 0m0.003s
4500000 0m2.014s 0m2.013s 0m0.000s


I ran the tests on both Firefox and Chromium to compare results


  • User Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Firefox/91.0
  • Hardware concurrency: 12
Difficulty Duration(in ms)
500000 401
1000000 413
1500000 398
2000000 394
2500000 1495
3000000 1556
3500000 3971
4000000 4235
4500000 4116

To be fair, my Firefox installation is loaded with a gazillion extensions while the Chromium instance is clean, as I don’t use it much


  • User Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/92.0.4515.159 Safari/537.36
  • Hardware concurrency: 12
Difficulty Duration(in ms)
500000 399.40000000037253
1000000 354.6000000014901
1500000 351.19999999925494
2000000 353.80000000074506
2500000 1337.800000000745
3000000 1311.199999999255
3500000 3417.5999999996275
4000000 3488.800000000745
4500000 3458.2999999988824


At the highest difficulty factor, the native implementation was a almost second faster than the WASM library. But the fact that both of them were able to run to completion in under 5 seconds is impressive!

So, in my opinion, native implementation is only slightly faster than the WASM library and for all intents and purposes, this shouldn’t matter much.

P.S Work is underway to benchmark multiple platforms. A detailed report will be published when that data is available.

For this post, I asked some of my friends to run the tests on their computers. The results slightly varied but even the slowest case generated proof for 4500000 difficulty(the highest in this test), in under 15 seconds!

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