I don't have issues logging in on a very full account, but it wouldn't surprise me that some people have issues of that sort. Once upon a time, Character data with lots of quest completion data increased zone-transfer times by minutes. And when the CP system was younger and IC was newly released, players with very high CP totals had issues with loadscreens as well. The transfer would get stuck on their very large number of passive effects, in some cases necessitating the player to either despec their CP or turn off passives in some way in order to pass a ladder. Taking armor off in order to deactivate gear passives was one method to mitigate the issues.
So, where does this problem occur? Initial account login? Zone entry after the character select screen? Area transfers after already logging in?