I recently started having this problem with any project I work on in Premiere Pro. Everything seemed fine before, when I started my project. It is high quality 4k files from a GH5 which was working fine from my external usb3 HDD. Now I reopened it and it was running very slowly, taking ages to generate the thumbnails in my video bin (not sure what changed maybe an update?). This seemed to be taking up loads of ram as when I didn't have my video bin open then this was not an issue and things were running much smoother.
I had two 1tb HDD's lying around so I put them into my computer and made them into a RAID drive which runs at nearly 200mb/s. This helped a little but it seems to be more an issue with ram. It will peak at 95% ysage then suddenly go back down again depending on if the video bin is open or not.
16gb DDR3 1600mhz RAM
256gb SSD System Drive
120GB SSD Media Cache Drive
1tb HDD Export Drive
2TB RAID HDD Footage Drive
GTX 970 GPU
I tried clearing the cache and then I moved to the above drive. Even after putting all these separate drives in it has not resolved. I can see in the Performance Monitor that the Footage drive is never going over 30% usage. It seems to be a ram issue as the CPU is not at 100% either.
Also I updated to the latest drivers for my GPU and the latest version of Premiere.