no no no no, and again no. having content in the content browser will not slow down the engine. you can measure this by building a small level, say a main menu with like 2 trees in it, then migrate that to a brand new project. set it to default open level in both projects and time how long it takes to load. it will be the same.
ue4 only loads assets that it needs to open the current level, it’s not like it’s going to load 20GB worth of assets to open a random small level you might have.
i have a testing grounds project where we take all of our assets to be vetted, it’s grown to over 120GB and takes just over 30 seconds to load the main menu level when opening the editor, once opened we can then load different levels for testing different assets.
the only time the ue4 interface will cause fps loss is when you have lots of different windows open or have them spanned across multi mon configs.
If you want to speed up content browser a bit and you are OK with outdated tumbnails, you can disable realtime tumbnail rendering (click eye icon for more options). With this disabled tumbnails should be refreshed when you hover with cursor over them. Other thing is you can lock max FPS (via INI config file and console command) for editor to value you are comfortable working with. Eg 60fps or even less if you wish.