Just upgraded to Premiere 2020 in our studio. Having an issue on our PCs. Premiere cannot playback 1080p @ 30fps JPG or PNG sequences at all. Audio will play but no update to the viewers. Exporting the sequences is successful but takes an extreme amount of time compared to other formats.
Turning off GPU acceleration will improve playback to 1-2 FPS. All of the above is with no effects applied, and with one video track only (the image sequence)

By experimenting with other formats I found that TIFF (no alpha) and EXR (with alpha) sequences playback & export just perfectly.

We have several older Mac Pros (2015 era) with AMD GPUs that do not have the issue above using the same PNG or JPG sequences.

Replicate as following:
Create New Proejct
Import PNG or JPG sequence
Create timeline with imported sequence
Hit Play.

Rough system specs :
I9-9900X
RTX 2080
32GB Ram
Win 10 10.0.18363
Latest Nvidia drivers (as of 12/1/2020), also tried installing the creative-ready drivers with no change.

Already did the standard Adobe 101 troubleshooting of deleting prefs & cache to no affect.