I used pnpm in the past too but found too. I like the idea but many incompatibilities made me ditch it. Last time I touched it was probably 1.5-2 years ago so maybe it's moved on.
Re. the speed thing, surely we're talking a few seconds extra build time for a the install process that's a small fraction of the overall build and deploy time though?
Haven't looked at workspaces, I haven't (knowingly) had the requirement for that, will check it out