Then you need to find out what exactly is going on.
Make empty project (use some default template). Check if there is FPS difference.
Then do same with some epic “demo” projects, something middle size.
And then some big one you can download and package.
From results of those you should be able to see if it depends of project size or not.
Remember you looking at % difference between editor and standalone, ie if standalone is slower than editor.
ps. I am guessing it may be some rendering settings, something that is turned off in editor but is activated in standalone. To check that you can make postprocess volume, make it unbound and overwrite project settings with it (set all effects in that volume to turned off.)