Did a stat DumpFrame -ms=0.1 Noticed that SlateDrawWindowsCommand was taking much more than I expected it to. Googled it and found this Bad Slate performance.
When I packaged the game t.MaxFPS 120 actually worked.
So it seems that the Editor is somehow syncing/holding back the GPU to specific refresh rates (30 and 60 fps). Would be great to know how to disable it.