Service Managers self service portal is probably going slow because you’ve got JIT (Just In Time) Compiling enabled in IIS. This basically IIS compiling the self service website before delivering it, which makes it deliver the page more slowly.
JIT Compiling is good for saving server resources, because application pools clean out pages that aren’t used regularly when the application pool is recycled, freeing resources.
To fix this, you can stop the application pool from recycling by:
Log into the self-service portal web server
Start IIS (Start -> inetmgr)
Expand the web server
Click Application Pools
Right click on the SM_AppPool (assuming you didn’t rename the pool when installing System Center Service Manager)
Click Advanced Settings…
Change Idle Time-out (minutes) and Regular Time Interval (minutes) to 0
Right click on the SM_AppPool again and click Recycle
The self service portal will go slow once, then be forever fast (as it will not be recycled, except until rebooted or if IIS is reset).