It may be a bad drive. Having this exact same problem, system extremely slow after Time Machine restore, all vitals check out fine, but I'm using a late 2015 iMac. Seems to be a pretty specific problem. It's important to note this is an APFS volume, as this seems to be somewhat common. I upgraded to Mojave about 3 months ago.
Opened console, and saw invalid btree errors constantly happening as files were trying to be accessed, I'm assuming that's the cause of the slowness. Disk Utility found errors but couldn't repair, booting in single user mode using fsck couldn't repair.
Decided to wipe the volume, and re-install the OS. 2 minutes before finishing the first step of the install, an error appeared saying there was a problem with the install, and to "try again." This was enough evidence to tell me this physical disk is bad.
One more thing to note about this, my Time Machine drive (external 3TB WD drive) went bad at the same time, cannot be repaired, doesn't show up when doing a recovery from a backup. Not sure what to make of that, as that's obviously too coincidental.