The authors of LTT claim that the performance hit for a patched kernel compared to a regular kernel is minimal; Their testing has reportedly shown that this is less than 2.5% on a "normal use" system (measured using batches of kernel makes) and less than 5% on a file I/O intensive system (measured using batches of tar).
The authors of LTT argue that the performance degradation from using a patched kernel is minimal; Their findings show that it is below 2.5% for "normal use" systems (as measured through multiple kernel updates) and less than 5% for file I/O intensive systems (measured by tar batch operations).