VSX is not really my area, but check these things:
1) DNS servers throughout the VSX system - test with nslookup and make sure all configured DNS servers are reachable and fast...this can cause a lot of startup delays
2) Single-core limits - I've noticed that the vast majority of unaccelerated policy installation time to a R81.20 cluster is spent with process fw_full (fwd) on the gateway pounding a single core to 100% and having to wait for it. I realize processes like this are legacy and single-threaded, but seeing this type of thing on a 32+ core system is maddening.
On your 16200's try running top with all individual core utilizations displaying. While overall CPU usage may look low, watch the individual core utilizations while a single VS is starting up. Is there one CPU sitting at 100% constantly that goes back to idle once the VS is finished? What process was causing that? You can do the equivalent of an strace against the process as described in my Gateway Performance Optimization Course, which may give you insight on what resource (file, socket, etc) was causing the process to get stuck:
Gateway Performance Optimization R81.20 Course
now available at maxpowerfirewalls.com