To start off, I have not noticed any slow downs while using a system loaded with tboot. I don't really know of any reason a slowdown would occur, but perhaps one of the other people on this list can offer suggestions from a lower level. As far as I am aware, once tboot puts the measurements into the pcr and hands off to the OS kernel, its role is complete.
Can you offer some specifics on your problem? Do you mean overall machine performance in the OS is subjectively "slower"? Are you talking about boot speed? Perhaps some more objective performance benchmarks before/after tboot installation would help show your problem. If you disable the TPM/TXT, do you still notice a slow down?