Zipping up the Lib directory from the python source (1735 files) as a test, it took on average 0.10 sec to read the zip before, 0.04 sec after.

(To test the time taken by zipimport isolated from other startup costs, instead of actually getting the zip in the import path, I just ran

import time, zipimport; start = time.clock(); 
zipimport.zipimporter("lib.zip"); print time.clock() - start)