If I want to delete particles/voxels from a FLIP sim (sop) during the simulation based on a condition (to optimize sim time), how can I do that?

For example, I know it is ok to delete any particles with speed > 2500. Obviously I can blast them post sim, but I'd much rather tell the solver it can stop working on those.

Eh, I wouldn't do any of that to optimize simulation time. If you have too many particles, remove them with a sink. Make your domain smaller. Do some shot planning with the camera so your simulation is optimized based upon what you're actually going to render.