Hello Phillip.
I can talk from experinece about this issue.
We had severe issues on the Dave Cullen forum with this matter. Forum loading time began to slow, and we even had a few crashes as well.
We soon discovered that excessive PMs were the problem. We found one member who had over 8,000 PMs saved!
Once people began to get rid of older messages, the forum's problems stopped.
8,000? That's like my Gmail inbox! I had upwards of 1,500 myself. I'm down to 800 or so. I am trying to purge a few dozen a day. I honestly read every last one of them, but sometimes a lot of my PM's are comments and input, which I don't always have an instant answer for, but usually those things get implemented down the road, so the input is absolutely honored and appreciated.
We've never really had any technical issues because of the number of PM's, beyond just keeping everything backed up. I suspect the DC issues may have had to do with the hosting company used and how much memory was allocated to SMF, which can be a memory piggy at times. DC is also larger than we are, so the specs of the server and the resources provided by the hosting company can also make a difference. What John and I have learned is that the best way to host an SMF forum is to place it on its own server, or use one of the virtual server technologies that are out there, which allocate "slices" of resources which are guaranteed to be exclusively available to the forum and no other customer. That way you don't run into a problem with oversold hosting companies or servers that, over time, simply have issues with memory leaks or other problems that require rebooting.
With SMF, the biggest reasons for crashes or slow responsiveness comes from:
1) the hosting company
2) installed modifications (mods) to the SMF basic software which are not well written.
Most crashes happen because of the hosting company. We used to go down about once a month until we finally ditched an older company and switched. We've been up for several hundred days with no problems ever since. And third party software mods can also be an issue.
Should DC wish to explore switching hosting companies down the road, you can PM me and I'll share our experiences and recommendations.