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1, Editor runs at 20 fps tops and eats the pc as it is while looking way to fancy? Can it be simplified to basic windows look? (Ryzen 5 3600, RTX 2060 Super...is this for real?)
Unreal is a bit of a hog. Almost any major game engine is. If you can get on an SSD, it will run better.
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2, I had to download 137Gb of data (most people don't need that EVER) then at startup it unpacks stuff for a long time, why not just unpack the game PAK itself locally? What files does it load, latest game version?
Welcome to working in Unreal or any game editor for that matter. When most game editors start up they have to do a bunch with assets. it's normal. This is just something an editor loaded with as many assets as MW5 or any other game would do. My advice is to start up the editor and then go chill out doing something else for a bit. Even the professionals start up their editors and go get a cup of coffee. It's just game development.
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3, Can not load existing mod pak files? If i want to update a mod for myself or rewrite it to run at later version i have to do that from scratch?
PAK fires complied files. You can unpack them. Google UnrealPak and you'll find a tutorial about unpacking the files. It's just a series of DOS commands in the command prompt. Once you unpack the files, create a junk mod and put them into a folder in Windows explorer for the mod you've made. Then, open the editor and load the mod and you'll find those unpacked files in your folder structure.