Optimization is an area of specialization, all on its own. There’s no way anyone will be able to teach you how to do it here.
Plenty of vids on youtube, and Epic training material about it also.
But, I will tell you a shortcut, that’s easy to do, and works. First thing is turn off auto-save.
-
If your level was empty, it would run fine, right? Try that. Delete almost everything and try it. If it runs ok, then it is something in your level, otherwise, it’s probably something you’ve put on tick.
-
Now start again, but only remove about half the stuff. Does it run ok? If yes, then you removed what was causing the problem. If no, then it must be what is still here.
-
Keep going like that until you hunt down what is causing your main problem.
Although it’s true, the more things you have in the scene, the slower it’s going to run, there probably is one or two big problem asset types. This method, will help you find them.