Downgrade from Mojave to High Sierra

My biggest concern is not losing any data - especially anything I've done since going to Mojave.

To accomplish that, restore the last High Sierra system that was backed up. Follow its instructions for "Restore both macOS and your files".

Then, selectively restore the data you require by restoring those specific files.

While you do that, I suggest temporarily de-selecting "Back Up Automatically" in Time Machine's Preferences, to preclude any possibility of a subsequent automatic backup overwriting the files you want to restore, should Time Machine require the space they occupy. Remember to select it again when you're finished.

Robbie28 wrote:

Is it possible to do just a restore of macOS and not the files?

Yes. If I understand you correctly, you want to erase your Mac completely and then install macOS "High Sierra" which is certainly possible. You would be following the instructions for selling a Mac (which returns it to its initial shipping state), followed by upgrading to High Sierra (assuming it shipped with a macOS version earlier than it).

First: What to do before you sell, give away, or ... - Apple Support

Since you're keeping the Mac all you really need to do is Step 6 (erase).

Then: How to upgrade to macOS High Sierra - Apple Support

I also don't know how to figure out what specific files I'd need to restore, since it's been months since I upgraded.

You would need to peruse your Time Machine backups and determine which files you need to restore. Backups are date-stamped. You can even choose to retain the most recent versions if you're not certain.

It might help if you were to explain your reasons for needing to downgrade, because I'm at a loss to understand why you would perceive that need. Perhaps there is a better solution for whatever it is you require.

Good thing I asked. Upgrading macOS does not cause a Mac to become slow, which means downgrading is unlikely to fix anything.

Back up your Mac if you have not done so already. To learn how to use Time Machine please read Back up your files with Time Machine on Mac.

Then: Please determine if the same problems occur in "Safe Mode": Use safe mode to isolate issues with your Mac. Starting a Mac will take longer than usual, and using in that mode for any length of time is generally inconvenient, but once it's up and running it should not run slowly. If it does, the possibility of a hardware fault (usually, a hard disk drive that has been operating in a state of failure) becomes increasingly likely. That's the reason for creating a backup first.

The good news is that replacing a failed hard disk drive is easy, should that turn out to be necessary.

I suggest you download the process at the link below. Run it and post a copy of the report here.

https://itunes.apple.com/us/app/etrecheck/id1423715984?mt=12

It has been my experience when Mac as slow after and upgrade, it is usually because of something is installed. It might actually be simpler to fix it rather than downgrading.

It also though my old Mcafee was causing the slow down.

It goes without saying that the first step in fixing an ailing Mac is to rid it of utterly useless garbage like "McAfee".

Often, it's the only step.