Software RAID is slower than hardware RAID any time the host's CPU is busy doing something else. Unless the system is a fileserver only, a system with enough storage to necessitate RAID is likely to have that problem fairly frequently.

On the other hand, software RAID does have the possibility of being faster than hardware RAID because it can use multiple data paths. But that doesn't happen until you're using a machine that has a multitude of HBAs.

On the other hand (foot?), undercutting all of this is the fact that we're probably talking about Windows RAID, which, from all accounts, is both slow and unreliable. I've also had problems with the reliability of Linux' RAID in the past which makes me trigger shy about it. I'm totally comfortable with Solaris's software RAID, though. Never had any problems that I didn't cause. (If you go that way for whatever reason, never do a reconfiguration reboot after you've got it set up. Always go for drvfsadm.)