/3/ - 3DCG

I have a question about blender.
I use Blender for video editing and nothing else, but what bugs me the most about it is that it's almost completely impossible to work with preview as soon as I add an effect like grading, compositing or alpha effects.
For transitions it's not too bad since they end, but when I want to grade an entire project, the preview-FPS drops to 3-0.5. I tried using proxies, 25% but it never changes everything, even forcing 25% replay in the sequencer preview properties.
Is there something I am doing wrong? I just check the 25% box in the preferences and click "rebuild..."