If you start in Frankfurt, the security with the general public even with “fast” track can be horrendously slow, and if you have to go non-Schengen, and if you are not EU/EEA, it could take a very long time too. FCT on the other hand unbeatable in terms of speed passing through security as well immigration. For me, if I’m in transit Schengen-Schengen normally I simply go to FCL (car transfer I really don’t care), if it’s Schengen to non-Schengen and have the time (esp. if the transit is from A to gate B/C), I normally would prefer to walk over to FCT (provided it’s not pouring like cats and dogs outside.

Having said all these, I usually do my best to avoid FRA anyway.