So like render the snare to audio, render the kick, etc. and then mix them all. Would that then reduce CPU usage?
It depends where you're doing any other processing on the kit.
By the time it's rendered (bounced to audio), CPU isn't an issue & the rendering itself is non-realtime, so it doesn't matter.
If you're using the kit plugin's own mixer, groups, compressors, EQ etc, then bouncing each individual instrument or mic will affect how the compressors, group busses & channel gains interact.
For instance - I use Toontrack's Superior Drummer.
I can bounce every single mic to its own track & then deal with it as though I had actually recorded all those mics dry & live.
This is a big job - 20 or 30 mics coming into the board, no EQ, no grouping, no sub-mix... it's "pure" but it's a lot of work. I've done it sometimes, but not often. If I do it this way the first thing I'm doing is patching up a lot of noise-gates to tame the bleed (spill), then dropping the right mics to the right busses. (Yes, It's a plugin, I can simply switch the bleed off entirely, but if you're gonna do it properly... do it properly ;)
However... I'm far more likely to simply render the plugin's stereo output - using the already sub-mixed channels inside the plugin, maybe some slight onboard EQ or comp... & then treat that as a stereo pair for the rest of the session. It retains a certain "live" quality to it, every mic interplays with the others, the individual close mics, the spill into the other mics on the kit & the array of overall ambient mics available. It gives a solidity I would take me an hour to get back with the individual mics rendered, even if I do really prefer my own comp/EQ to those in the plugin.
If I change my mind, I can just trash the bounce, tweak & bounce it again.