There can be many different reasons, prcan. But the time frame is consistent with some other problems on .com forums.
I will report this to the Webmaster. But he's got a few urgent things at the same time right now, so it may take a while.
Meanwhile, you could try some things to lessen the problem:
Go to the Admin Panel >> General Admin >> Configuration.
And scroll down to Topics Per Page and Posts Per Page and lower them.
You could also try to switch style template, to the basic subSilver, and set Override user style to Yes.
This may or may not work, depending on the source of the problem. But it is well worth a try, if it makes it a little more comfortable for you and your members.
FoxLeonard