We have a fairly large instance that has been live for about 8 years.
We have roughly 30,000 users in Eloqua, however only about 15,000 are actually live (mainly for Engage and Profiler users.
Some areas in Eloqua take forever to load because they must load all the picklist values for users, for example the picklist for email test or anything allowing signature rules takes at least 30-45 seconds to load. Our other instance is light, with only a few hundred users and it is amazing how much faster and more responsive it is. What i believe is happening is the UI must load every user from the table if they are active or disabled which is silly, why would you need a disabled user in a picklist?! It is creating way too much overhead!
My suggestion here would result in a major performance improvement throughout the platform for end users.
The solution is to simply HIDE disabled users!