There are a couple of tricks you can use to make JSPWiki faster if you have an Apache front-end (either using mod_jk or mod_proxy):
The easiest trick is to enable compression using mod_deflate. For example, put the following in the relevant configuration file (e.g. deflate.conf)
JSPWiki does ship with the CSS and JS files compressed already, so the saving on those is not that big, but the saving is significant for HTML.
A lot of data is not changed very often (for example, images and other static content). Therefore it makes sense to set the Expires-header on this content. JSPWiki does not set it automatically. Even if it could, it's probably better to have the web server take care of this sort of stuff anyway.
For Apache 2.2, you can use the mod_expires library, which allows you to automatically set the expiry times. Here's a good config. If you change your Wiki a lot, then you might want to have shorter expiry times.
Put these in your httpd.conf or expires.conf.