Apple has gone and done something really wrong in terms of security: they released a critical security update wrapped in a feature update.
So, Quicktime 7.5 is required to be protected from the most recently disclosed vulnerabilities. Problem is, as with every other n.X release of Quicktime, it’s buggy. No doubt 7.5.1 and 7.5.2 will be along in a few weeks’ and months’ time, but until then your only choices are to run with miserable choppy playback or to stay vulnerable to disclosed security problems.
This is a really bad idea. There should have been a 7.4.x rev for security as well as a 7.5 with those security fixes.