I would suggest looking at your admin panel for this forum software and see how caching and buffering is being handled. Many really active sites can get choked up during peak activity times because of all the update pointers for each and every user reading a thread that has those "last read" pointers stored in the database.
Are you leasing your own server, or being hosted by a third party hosting company?
If you are leasing your own server, talk to your server admins. See what sort of buffering is being used for the base server software (usually PHP) and ask them to up the amount of memory used for that function. Too little memory causes too many page requests to be stored on the hard drive, which is MUCH slower than system RAM. If you have an old server, consider upgrading to a new machine that uses M. 2 NVMe SSD for the OS level software.
And please, analyze your ad loading. Especially dynamic ones that will refresh with a new ad frequently in the same zones. Each and every one of them will be stealing bandwidth from your viewers. Static ads only need to be loaded once and are a lot less of a drain on the bandwidth. Bear in mind, too, that every ad has a cost to you in bandwidth. Yeah, it is great having income from them, but there is a point of diminishing returns that can be hard to recover from when you start losing participants.
IMHO.