This Dell Latitude XT tablet's SSD drive has slowed quite a bit lately. I've read that you can do a drive image and complete wipe as one method to speed up the system, but that seems really tricky given there is no CD/DVD drive in this machine. Can I defragment the drive and wipe the free space somehow?
It seems unlikely that you could do a selective wipe of the free space if the drive doesn't support TRIM, because TRIM is the standards-compliant command that does the selective wipe of a portion of (rather than the whole) drive.
There may be a vendor-specific way of doing it, you'd probably be best off to contact the drive vendor's support about it. Worst case there may be a firmware upgrade that allows this drive to support TRIM.
Another option you may want to consider is replacing the drive with something that has better firmware including TRIM support and possibly firmware that doesn't really need TRIM. Many of the current crop of drives keep a pool of unusable storage and use this to as a pool of erased blocks so that TRIM isn't as necessary. I have some Intel X25-M drives which do this and they just work -- no babysitting required. :-)