Purge old tickets or database maintenance?

We're at just over 200k tickets after about 2.25 years of using WebHelpDesk. We've had some issues with slowness that started to crop up once we hit the 200k mark, so I wanted to check about any procedure to purge old tickets. I can't necessarily blame ticket count on the slowness, as we do have quite a few Techs using the system. But I'd like to know my options, especially as we're a Public organization so it will probably end up as an Audit question...