After spending too much time today waiting for WebSphere Portal to restart (due to a nasty bug causing Portal to crash when I was working with it) I've been revisiting how to improve the startup time.
The key step in doing this is to uninstall portlets that aren't needed. (see http://www-01.ibm.com/support/docview.wss?rs=899&uid=swg21213982&ca=lsall for a list). For the lazy developers (read me) there are a series of scripts that have been developed that do this. The article Installing and configuring WebSphere Portal V6.0 Servers for development with Rational Application Developer V7.0 and Rational Software Architect V7.0 includes these scripts in a zip file (search for StartupPerformanceConfig.zip in the article). The article also has some good suggestions for memory and jvm settings for your server.
I have known about the existence of this process for a little while, but it took me a little to find them just now (when tuning a development server). Google was have troubles pointing me to the right article, so I thought for my sake I'd try and help google find this in the future. (Even if google did find the right spots, the title of the developerworks article wouldn't have made me think that it had the nugget of information about IBM WebSphere Portal Startup performance).