Problem reproduces in every version of the engine back to 2018.
Problem reproduces in 2020.1 on SRP master (commit 8823d0532c) but will not slow down as much (and requires more tabs closed to notice the effect) due to optimizations made to shadergraph.
Problem seems to be exclusive to shadergraph. VFX graph was tested and the bug did not reproduce.
Problem is exacerbated in perceived frequency in 2019.3 and above due to the fact that shadergraphs now automatically dock themselves when you open them.

Repro steps:
1. Use the default layout (may not be necessary but it is what I am using)
2. Repeatedly open/dock tabs to the main section where the scene and game tabs are, and close them. Close at least 10 tabs this way. Easiest way to do this is make a bunch of new shadergraphs and open all of them (on 2019.3) and then close all the tabs (via right click). In versions before 2019.3 you'll have to dock them before closing them. Alternatively, you can close the "game" tab and repeatedly open it by going to window -> general -> game and close it again.
3. Open the attached shadergraph and make sure it's docked to the same section.
Expected result: the shadergraph runs smoothly.
Actual result: the shadergraph runs extremely slowly.