SCSI versus IDE hard drives

I would like to hear your opinions as to the pros and cons of these two types of HDD.

My understanding was that SCSI drives run faster than IDE drives. However the only Windows computer I have that uses a SCSI HDD runs very slowly. Perhaps this is a software issue, but it seems to respond very slowly to various commands, and especially when downloading web pages.

And no, there are no viruses present.