Yes, a full-sized nut in steel or bronze will have lots of inertia, which contributes to misfire and loss of efficiency. Try reducing width by about a quarter. Of course if the nut gets smaller (either width or diameter) the fit to the nut-socket becomes more critical. Otherwise it can oscillate heavily in the socket. Mounting the nut on an axle will reduce oscillation to almost nothing, but then you've got an axle that will eventually wallow itself loose in its mountings, or bend in the middle, binding up the nut. Yeah, there are no easy answers. Anyhow, a smaller nut oughta help.
My first crossbow had a huge brass nut, made to Payne-Gallwey's specs. Jeez, that thing was heavy and slow for an 80 lb. Wham-O prod. Eventually, I drilled a bunch of lightening-holes in the nut, increased the hollow in the backs of the lugs and tightened up the socket a bit. That first bow still misfires occasionally if used with a bolt-clip. Without the clip it shoots fine.
As noted before, aluminum will make a strong, fast, lightweight nut (fitted with a harder sear of course) but aluminum has its own set of problems. I can't recommend it.
You can make your own nut out of brass round-bar with hand-tools, though it will take a while. Steel is stronger and lighter, but you'll probably need access to a milling-machine.
So yeah, in short, make the nut narrower... maybe a bit smaller in diameter.