I also find it very slow and in fact that is the last thing that keeps me from rolling it out in our company.
I've already split up my project into smaller projects/stacks but now I'm at a point where it would make it too complex to split it up further. Because when using multiple projects you have to somehow manage the dependencies between those projects and update child projects if parent projects changes. The Automation-API may help here but you have to implement dependency management yourself.
Maybe I've missed something here.
The Pulumi web portal also gets very slow as the stack gets bigger and every click is becoming annoying.