MUGSI gets easily overloaded. It happens every year. Best thing to do is to login at odd times like... really early in the morning when most students are sleeping in
(usually if you go on before 9am its not busy) or going on later in the evening after dinner time (after 4pm is usually when it frees up again).
Also, note that the scheduler, timetable, and mugsi are on three separate servers. So when you go mugsi.mcmaster.ca, first try your "Undergraduate Personal Timetables" which will show you the scheduler. If that's not working, then try the McMaster Timetable system (which also shows you the room number but takes longer to look it up) -- at
https://adweb.cis.mcmaster.ca /mtt/