It's always been that way, and the rendered version often causes problems because PD will try and use it even when you've made further changes to that clip on the timeline.

You can easily delete the cached versions as described here.

I've found that it's much quicker and far more reliable to use the Range Selection markers to select a section of your timeline then use Produce Range to fully render it. Replace the timeline section with the newly-produced content and PD won't slow down because all the cumbersome edits will be fully produced.

Make sure you produce any sections in the same format as your finished video so PD won't have to produce those sections again. Depending on the format and what other edits are involved in the final project, SVRT may be available which can really speed up the final producing run.