I wasn't able to access the internet yesterday owing to a family problem. However, I do get notified by email and SMS if there are any server issues of this nature. Normally, the only time the server itself runs slowly is when full backups are in progress which are timed for the early hours at weekends and even then it is rarely perceptible. A few months ago we did encounter some unexpected heavy loads on the server which could not be explained but (as is always the way) ever since monitoring processes were put in place to identify the cause, everything has been 101% ticketyboo.
The only other thing within my zone that can cause slow access is if one of the two "pipelines" to the server (which are with different providers) is out of action through failure or maintenance; this causes all traffic to run through the other one which, at peak times, can limit the speed of traffic despite the server itself running at normal speed. I haven't had any reports of such outages (planned or unplanned) for many months.
A few weeks back I did find one user's avatar, which was on a remote server, gave major problems with loading some pages. This must have been a configuration problem at his end as an image shouldn't normally do that. Disabling it solved that, along with a suggestion the image can be uploaded to our server which would avoid any such issues.
Occasionally slow running can be caused by inexplicable local circumstances. It is always worth both clearing your browser cache and re-starting the computer if you experience such problems. Re-starting the broadband modem often helps, too (this causes you to connect through your ISP through a different gateway). I discovered the value of the first of those three with two computers in the same room responding completely differently until I cleared the cache on the slow one.