That's an interesting question which deserves a three part answer.
The forum database only consumes about 80MB, so for as many posts as there are the forum doesn't eat a lot of resources. As I've found with the Amazon Kindle, text is small (I have over 200 books on mine now and they only take up 60MB, just as a point of reference.)
The second part of the answer gets a bit technical but I'll try to keep it lucid. Back in the days of yore, memory (RAM) was way more expensive than disk, really expensive in general, and tended to limit the size of programs people could run. Some bright guy figured out that you could copy the contents of memory to disk and swap stuff back into memory as it was needed, thus making the computer seem like it had more memory than it actually did. Thus "virtual memory" was born.
There are some downsides, primarily that it takes time to swap data from RAM to disk and back again. This leads to some interesting discussions about trading time for space, a key concept in optimizing systems. There's also a limit to how much virtual memory can be used, and people with really big brains spend a lot of time figuring out the best way to tell which data should be swapped out to disk when - stuff that's not referenced very often (LRU or "least-recently used"), etc. It's a pretty fascinating subject if you're into that applied mathy stuff.
Fast-forward to today. Disk is disgustingly cheap, RAM is almost as disgustingly cheap. Processors are even more blindingly fast. The time lag in swapping data from disk to RAM is really noticeable, so the goal is as it has always been - stuff as much RAM in the box as you can afford. The goal is to have enough RAM so that you rarely use the virtual memory (disk.) For example, running backups at 4am. It doesn't matter if the system is a little slow since most sane people are asleep at that hour.
To get back to reality, the server running the forums has 1GB of RAM and is using 200MB of swap. I've had another 1GB of memory sitting on my desk for a few months because I haven't felt like taking the machine down to install it, notifying people so they don't freak out, etc. I checked
the graphs and it looks like I've rebooted the machine three times this year, the last time was 11 weeks ago (the previous uptime record for the machine was ~250 days; I've run some servers for over two years straight, which is pretty good considering they live in my back bedroom and not an actual datacenter.)
In summary, the forum database is small enough that it doesn't desperately need pruning, and the system is slow overall is due to the use of virtual memory (aka swap space.)
But all that leads to the most important question: How do people feel about pruning old posts from the forums?
It's just the flip of a switch in the software and it'd take less than five minutes to do, but I'm wary of doing it because I haven't talked to anyone about it. There are some things I'll do out of sysadmin fiat, for example, taking the system down to put in memory, adding anti-spam features, deleting abusive-looking accounts, etc., but messing with forum content is something I don't do lightly. I've considered culling old posts (older than 12-18 months) in the time-sensitive parts of the forums - Casting, Corkboard, Events, etc., but only because I don't think anyone would miss three year old casting calls. Still, I need to know what the consensus is before I start making changes.
I should post a poll in the General Discussion topic; I don't know how many people watch the Tech Talk topic and I want some feedback and proposals before I do anything I'll regret.
Thoughts?