My host has tweaked the server as much as he can to ensure that it is performing optimally. We've upgraded the hardware twice, which made a small improvement each time, but not enough to warrant throwing even more hardware at it. We reboot it every few days as there is a very noticeable slowdown as time goes on. I also manually optimize the database every evening before going to bed. These two practices seem to help the most.

My host always points to the caching as a big problem. Part of the tweaks he did to the server were aimed at improving the cache system. I don't know enough about that stuff to tell you exactly what he did.

Thank you both for your replies.

Sounds like the hardware is inadequate - if you've upgraded twice its probably lacking RAM. If the 'caching' is an issue its probably sub-optimal hardware/mysql setup. Its possible there is some kind of SQL issue with MLF and large databases ofc. Although there are other sites with huge databases out there. You mentioned rebooting which, assuming its a LINUX/BSD platform, means its running out of memory. I think your only real recourse will be to go to a dedicated machine with the right software and hardware specification for the site utilization.

Good luck with it anyway.