For me, that emulator seems to drop about 1/3 of all keypresses at normal typing speed, making it basically unusable.

Yes, but the ZX Spectrum membrane keyboard and the PLA scheme for sensing one keypress among each set of five keys, also dropped keypresses and forced you to type really slow.

That means it's an accurate emulator! There's a reason Speccy had a word-per-key input mode for programming

I mean this in all sincerity, but just type slower.