Not only is it slower than chipmem, it is MUCH slower than chipmem because of the poor performance of the PCMCIA bus. I also heard that there are general system speed issues due to managing the bus. The latter is probably limited to network cards though.
Note: FLASH is very fast compared to a HDD for booting, even if you have a nice highspeed IDE buffer. There's just so many small files and paths to lookup that the seektime of the HDD reduce actual transfer rate far below FLASH.
A year ago I experimented a little by archiving some stuff without compression and just blast it up to ram: from HDD at startup (like env:). It worked so well (speeded up boot multiple times) that I desided to leave it there.
