How to fully optimize RAM Disks in Adobe Premiere CC 2017
I would think that the codec of your proxies could be the issue.
H264 is very cpu intensive and this could be your bottle neck.
Switching to QT ProRes Proxy would be my first try.

A couple of links to help you out.
https://forums.adobe.com/thread/2287035
https://forums.creativecow.net/thread/3/994398