Intel8080 | Intel8080 emulation in JavaScript

You're creating a new surface and a new texture every frame. Yeah it's going to be slow, creating textures is a ridiculously slow operation. Don't do it so often.

If you need to write pixels directly, create a texture of the right size once (with SDL_TEXTUREACCESS_STREAMING) and then use SDL_LockTexture and SDL_UnlockTexture to update it (note: don't use SDL_UpdateTexture for this, it's not really much - if at all - better than recreating the texture performance wise). I'm guessing the windows version is fast "by accident" because your driver implementation notices you're doing something strange and quietly ignores what you're telling it to do and does something faster.

Of course without looking at the code it's hard to say this is the only problem with it, but this is pretty likely to be a (or maybe even the) bottleneck.