If I may add, there's another forum thread on this same issue:
https://devcentral.f5.com/questions/portal-system-very-slow
There are a few options for acceleration. First, each portal resource will have its own compression and caching settings. Second, and this is a method that I tend to prefer, I'll build internal VIPs for each of my applications and then point the portal objects at these VIPs (basically VIP targeting).
Some level of web acceleration should definitely help, but aside from that, I would do a Fiddler/HTTPWatch capture of direct and portal access to each of these applications and compare the times for each request/response cycle. If you notice that large files, specifically JavaScript, CSS, Flash, or Java files, then that's probably the issue. The portal rewrite engine has to parse each of these files as they pass through the box, looking for links to rewrite. If the files are large, that can eat some time. There are definitely ways around this, but do the analysis first.