On the Collections,Can you please plot the time taken by PDP or PDP Workers, Refresh Snapshot or ODS over few days. If the behaviour is not consistent, then you may try running the GSS on ERP instance and VCP instance at 50% or may be at 100%.
Are there any other activities like backup being scheduled at the same time the Collections and plans are run?
You may also monitor the DB sessions and see what are the other processes/concurrent programs that are running at the same time.
We faced similar scenario in our environment. Below are the things we tried with success.
Increase memory Based Snapshot workers.
Enable MRP_DIRECT_LOAD-profile option that can be set at site level
Improve the performance of Loader Worker Program by implementing sql loader profile.
The performance improved considerably.