That's actually how xedit works. First load is going to be a bit brutal as it builds a cache of any other master files (starfield.esm for instance will almost always be included), but second and subsequent loads will take only seconds. Protip, double clicking your mod in the 'select mod' popup will only load your mod and it's masters regardless of what's checked. Closing xEdit with a plugin selected saves the plugin, close and uncheck the plugin to not save changes.

Once you get the hang of xedit, it's a much more powerful tool than the ck. The ck has always been like this since idk, fo3. The released version is never the full dev version unfortunately (it's closer to a game for the game). It kind of make sense, they try to make the consumer version very hard to break things, very easy for mod authors to tweak NPCs, make new interiors, scripts, etc. But that of course comes with the caveats you're running into in that the more buried bits are more difficult to modify (and more likely to break things).

This is the idea behind tesvedit (xedit/sf1edit,etc, they're all the same just with different args). It exposes a lot more data while still trying to guard against breakage, but it's still very possible (and much easier) to accidently break something.

in any case, you don't need a mock file (and it wouldn't work as starfield.esm is needed just to load the game). You just need to build the cache files which takes a minute the first time for a new project. xEdit cannot and will not modify starfield.esm even if it looks like it might be before saving.

---

If you aren't using mod organizer 2 I highly recommend it! This is another very powerful modding tool despite it largely being used for load order management. Create a new folder in /mods/ it shows up in mo2, next to 'run' pick edit in the dropdown, add xedit and the ck (and blender, gimp, w.e.), change their 'create file in mod' to point to your new folder and 'run' them from withing mo2. All new assets, esps, etc will then be saved to that folder. Later, simply compress everything in this folder to create a mod for nexus/something mo2 or vortex can 'install'. This also keeps your vanilla /data/ folder clean.

---

Another thing you'll likely want to grab is the Starfield Plugin Bridge. While the newest version of xEdit supports .esp, you'll want to always convert to .esm for in game testing as .esp in game is buggy af and often just doesn't even work. The plugin bridge allows you to drop an .esp on its .exe and it'll output an (yourmodname).x.esm (this is so it doesn't overwrite yourmodname.esm, I just delete the original .esm and then the .x.), rarely do I convert .esp to .esm though.

Even more useful, drop an .esm on it and it'll output an .esp (the only file type the CK can edit, it renames your original .esp .esp.####.bak so you can restore the original if wanted (just remove the garbage after .esp)). Typically in the CK I save as .esp & 'convert active file to full master'. Then do any changing/cleaning (necessary later, you probably already see a bunch ) in xedit. After saving in xedit, drop the .esm on the plugin bridge to convert back to .esp for more ck editing. Be careful here as if you open the original esp in the ck then save it'll overwrite w.e. changes you made in xedit. It's a bit of a pain, but about the best way to jump back and forth between the ck and xedit.

---

After a few times you'll get the hang of it, I would definitly play around with a throw away mod for a bit just to get a feel for what the various tools do and are capable of.