Bad Timeline Performance in Premiere Pro

Hi guys,

I'm working on .mp4 files in Premiere Pro and my timeline performance is getting more miserable by the day. There wasn't a sudden performance drop, it rather seemed to get worse with every Premiere update, or Windows update? - I can't say.
My usual projects are about 30min-2hours material in 1440p, mp4.
I'm using an i7-7800X, GTX 1080 and got 3 SSDs for system, video files and premiere cache on Win10.

Premiere Pro abuses my CPU when working on simple transformations on the timeline, causing 100% CPU load for 2min just to play one transformation fluently in the 1/4 playback window. This used to work with no delay at all, in 1/2 quality.
I activated Mercury playback-Cuda but Premiere doesn't seem to use my GPU at all.
Also I often read about Premiere using loads of RAM - I offer 50Gb and Premiere seldom uses more than 6GB.

I'm thankful for any help!
Got the feeling I am doing something very wrong?