This is super fun to play with, but the delay between the key press and audio playback is killing me. Are the sounds loaded into a buffer? Perhaps this is the best latency you can get with Web Audio (on Linux). Also, the semicolon key is not working for me. A fun toy nonetheless!
This will probably be about as responsive as this can get without a low latency audio driver. At least on Windows, this is similar to the latency when using a MIDI keyboard and a DAW with non-ASIO/WaveRT audio drivers. I think for Linux the analogue would be higher-latency with PulseAudio than with JACK.
I use PulseAudio on Linux, so that must be why. Is it possibly to get Firefox on Windows to use ASIO4all?