The C vs Firefox results are about the same that I'm seeing in my 8-bit emulator (http://floooh.github.io/virtualkc/). For the Amstrad CPC (currently the most expensive system) on my 2.8GHz Core i5 MBP I'm seeing about 1.3 to 1.5 ms 'emulator time' per 16.6ms frame for the WASM/asm.js version on FF Nightly, and for the native version (clang -O3) about 1.2 to 1.4ms.
This 'core emulator loop' is pretty much 100% integer code, with a lot of 8- and 16-bit integer bit operations (shifting, masking, ...). No calls into the browser or operating system, and almost no calls into the CRT or C++ stdlib (may be a small memcpy here and there).
The performance differences for 'pure' C code between browser- and native-version are so small now that they are no longer relevant, at least for the use cases I encountered, or rather within the normal performance differences when running the code on a slightly different CPU.
Calling out into HTML5 APIs is a whole different topic though ;)
This 'core emulator loop' is pretty much 100% integer code, with a lot of 8- and 16-bit integer bit operations (shifting, masking, ...). No calls into the browser or operating system, and almost no calls into the CRT or C++ stdlib (may be a small memcpy here and there).
The performance differences for 'pure' C code between browser- and native-version are so small now that they are no longer relevant, at least for the use cases I encountered, or rather within the normal performance differences when running the code on a slightly different CPU.
Calling out into HTML5 APIs is a whole different topic though ;)