Perhaps I can help here. I have a very beefy setup and I too have performance issues with manipulating higher res footage.
Here's a quick way create proxies after the fact.
Go to your bin and select the clip (or all clips for that matter) and right click on them. When the menu pops up, go about halfway down to the word "proxy".
Select "create proxy". You will then get this dialog
Here you can choose the size, I like the 1280x720, but you can go higher or lower. You'll notice that you can create and use your own "ingestion" preset.
Once that process is done (and you can still edit while this is being done in the background) you will want to go to both your Source and Program window and add a toggle button so that you can decide which you want to watch. (while editing I keep it on full time in both, but sometimes like to turn it off in one of the windows if I want to check details, like focus, etc)
Once you hit plus, you will get these icons (functions) that you can add next to the default set.
You will need to do this for both the source and program monitor separately. Grab the one I have highlighted and drag it next to the others and you're done.
Now as long as those are chosen in your windows playback and filter, etc will happen in real time. And, as mmarian said regardless if they are on or off when you go to export it will go back and access the original files and add all associated F/X.
Hope you found this helpful, I live by it as the first thing I do with all of my clips before I begin my project. It really speeds things up and no CRASHING!
Good luck, rb