I have a similar issue, but the lag is constant from the start when running around in the level.
I have some code running relatively intensive calculations on background threads before returning the results and doing some work on the game thread.
But the weird thing is that if I open a window (plugins settings, project settings or even just a prompt that I need to restart the project for some changes to take effect) and keep it visible the lag goes away completely.
The bottleneck according to the performance graph when lagging seems to be: “Draw”.
Any ideas to what may be causing this?
(On 64 bit Windows 10, rx 7900 xtx)