Freezing when fast previewing with J/L buttons

Trying to edit a trailer for a feature film and have some long, big file of ProRes4444 containing the whole thing. If I click for example "L" button 4-5 times the images start to freeze and I can't see what's going on and get lost. Pretty annoying. There's kind of delay when I stop. No problem in FCP7 with the same video. What could be the issue?