Hey Christoph, I use TFF on a 400 MHz G3 Pismo with 1 GB RAM. It's pretty
usable there. The thing is: You need enough RAM (512 MB is the bare
minimum), and this is even more vital if the HD and the processor are slow.
I'd say 1 GB or more is recommended. Also, you need to wait a minute after
the browser is started (look at the processor time in Activity Monitor)
because it takes a while to load completely in the background and will only
then be usable at full speed. In the realm of Clamshell iBooks (300/466 MHz
max.) 'full speed' still won't be very impressive, though.
The biggest problem with clamshell iBooks is RAM. Since you can't add more on these beautiful machines, you're out of luck here. The situation can, however, be mitigated: