Speed up get_user_instances

Fewer queries, simpler operations

Shaves off about 20-30 ms on an admin user in my dev box with 15
users. Makes the query timing waterfall diagram more balanced, for what that's worth.
Defer evaluation of querysets

This results in an additional speedup despite the additional (fast)
query, probably because these querysets are being modified again anyway before being
re-evaluated for rendering.