JBossAS betas are notorious for being slow to start. Even the candidate releases are often slow. Speeding up the performance usually happens closer to GA time, at which point you will probably see a similar startup time as 4.2.x.
By the way, my PC at home is dual boot. In XP, it take 90 seconds to start beta3, but in Ubuntu it takes only 45 seconds. Guess which one I use?
We did some fixes in the AOP and VFS subsystem that brought back the boot time for AS5 back to 4.2.x levels.
You just have to wait a couple of weeks for AS5 Beta4.