Thanks for the response.
I think there has been a bit of confusion. The issue is that the project is running at low FPS when using the editor not just when playing the game in PIE.
Also this has not been an issue previously and even if there is a performance difference between standalone and PIE, the difference would be way less than what I am experiencing currently.
This thread also mentions a similar issue. - Bad Frame-rate and Frame-time in Editor - #3 by J5689.
I tried changing the G-sync settings and it did initially work, but now the editor has returned to 15-30 fps again. Still I think the solution will be something along these lines.