This would be a poor idea overall. Vendor implementations of RAID vary, of course, so you will get different behavior with different vendors. Most assume that drives are significantly identical, so aren't designed to handle disks of such different performance characteristics. Chances are very good your performance will be handicapped by the slower disk.
For mirrored RAID writes, this is especially the case since many vendors don't consider a write Committed until it's on both disks, which will limit your write performance to the speed of the slower drive. For vendors that use a "commit when one is written to" you're going to be limited by the size of the write-cache of the RAID device, as that's going to end up buffering all the writes the slower drive can't commit to.
For mirrored RAID reads, you will still be limited by the speed of the slower drive in many cases as RAID vendors can do a couple of things:
- Pick One To Read From. If it picks the slow drive, all reads will be slow.
- Round Robin. Again, reads will be handicapped by the slow drive.
- Read-combining optimization, alternating. Once again, reads will be handicapped by the slow drive.
- Read from the drive with the smallest write-queue. This is the only method where the SSD would help.
Parity-RAID has much the same problems.