My understanding of the audio system is that each 'tag' has its own effects chain, thus making each tag a separate 'track', so to speak.
So, if you ganged together most of your sfx under a single tag "sfx", even an expensive effect like convolution should be pretty cheap.
FYI, audio effects are generally very efficient, and run in a separate thread from the main game, so they shouldn't really cause slowdown. Convolution is probably the heaviest effect in WebAudio outside of a really high rez fft; most everything else would barely register unless you are targeting mobile.
IIRC, the length of the convolution impulse sample will determine the cost of the effect; longer sample, more cpu.
Slightly OT: Oh, and don't use the Doppler effect; it's been depreciated: https://groups.google.com/a/chromium.or ... 1SI1GoHYO8