Inner Tube runs slowly on my secondary monitor in a two monitor set up.
Complex real-time 3D games are generally required to be run on the monitor set as your "Primary" monitor. If you have a fast enough system (Pentium IV 3 Ghz) with a good recent ATI or NVIDIA system it may run well even on the secondary monitor.
You can either try lowering the resolution of your secondary monitor or switching the primary and secondary monitors. You can also drag the taskbar and icons to your secondary monitor after you switch which is your primary and which is your secondary monitor.