Probably some assets being loaded at startup when they shouldn't be. Per moppius:
To get more info, run Unreal Insights before starting up the editor, with -trace=loadtime on the editor command line, then you'll get a list of all the assets and what's causing them to load, so you can sort out the low hanging fruit early. You can also do loadtimes.dumpreport in the console after loading the editor if you just want a logged list of the worst offenders (this is often where I start since it's very easy to do at any point)