Adding info from my own experience with this issue, since I just noticed it yesterday.
Note: I don't experience the monitor flickering that OP mentions, but I do get massive frame-rate drops. Tested just now with a fresh character with first stash accessible at the Traveler's Camp location.
When I first open my inventory, there's an initial frame drop of 20-40 fps due to loading the stash assets, but this stabilizes back to 144 fps within 1 second. However, I have a stash tab full (full) of 1x1 idols. When I switch to this stash tab, frame-rate drops to ~57 and stays there until I leave that tab again. I've tested this with other tabs as well, and there seems to be a direct correlation between # of items in the tab and the amount of frame-rate drop. For example, switching to another (full) tab with 1x2 idols, frame-rate stabilizes higher at ~91 fps, but still far from the ideal 144 fps I get elsewhere on tabs with fewer items in them.