If you have that person use Firfox with FireBug installed, you can get a timeline of all the requests and how long they took. That might tell you something about what's taking so long.
ETA: and if it turns out to be the main page that's slow, maybe try turning on debug for that user (using one of these methods
https://vborg.vbsupport.ru/showthread.php?t=265999), then scroll down to the bottom of a page and click on the ? after the number of queries and see if one of those is taking am unusually long time.