Performance is awful since upgrading to high sierra

Most likely the problem is with the system you upgraded, not the upgrade itself.

Possible Fixes for High Sierra

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Shutdown the computer, wait 30 seconds, restart the computer.
2. Resetting your Mac’s PRAM and NVRAM
3. Reset the System Management Controller (SMC)
4. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
5. Repair the disk by booting from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volume entry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and return to the Utility Menu. Restart the computer from the Apple Menu.
6. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button under Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
7. Download and install the macOS High Sierra 10.13.2 Combo Update.
8. Reinstall OS X by booting from the Recovery HD using the Command and R keys. When the Utility Menu appears select Reinstall OS X then click on the Continue button.
9. Erase and Install OS X Restart the computer. Immediately after the chime hold down the CommandandRkeys until the Apple logo appears. When the Utility Menu appears:

1. Select Disk Utility from the Utility Menu and click on Continue button.
2. When Disk Utility loads select the drive (out-dented entry) from the Device list.
3. Click on the Erase icon in Disk Utility's toolbar. A panel will drop down.
4. Set the Format type to Mac OS Extended (Journaled.)
5. Click on the Apply button, then wait for the Done button to activate and click on it.
6. Quit Disk Utility and return to the Utility Menu.
7. Select Reinstall OS X and click on the Continue button.

It's normal for your computer to be slow immediately after an upgrade because spotlight is re-indexing everything. But if that continues then you may have some other problem.

try looking at the activity monitor to see what's chewing up "energy" and "cpu" or if the memory pressure is high.

if nothing there then try looking in the console.app to see if it's spewing error messages.

create a new user, reboot the computer, and log in to only the new user. Does the problem go away. if so, then your old user may be autolaunching some program in the background or have wonky preferences and permissions that are scrwing up the apps.

if nothing is coming up then boot into "safe mode" (shift key on most macs). this will have lousy graphics performance but don't mind that. Just see if the problem goes away. If it does then either you have a hostile third party kernel extension or some other daemon is screwring things up.,

Assuming you don't have a weird kernel extension installed then the only thing left here is the hard disk or it's interface. (well possibly some other interface could be bad, like graphics, and show up this way but that's getting into the voodoo realm of mysteries.) So One more suggestion. Open diskUtility and check the drive's S.M.A.R.T. status. If it's anything other than Normal (i.e verified) and then replace your hard drive. You cannot fix this-- the drive must be replaced if it's not normal/verified.

if that shows nothing then something On some macs you can boot into hardware diagnostic mode. I think it's command-D or just D held down at boot for many models. I don't recall this moment what it is for yours.

After that I'm tapped out of ideas

In the realm of voodoo, if your processor's heat sink or fan is bad then the processor will slow down to protect itself. It won't tell you it's doing this-- I had one macbook have it's fan die so this does happen. There are various tools you can download (app store) to report the internal temperature and fan speed. The one I have used are names SMCfanControl and iStatPro -- it used to be free but not any more. But there are probably some others that are free still.

Might want to check that you have all the memory available that you think you have installed. if one of your chips died in some clean way that didn't kill the computer (normally bad memory == no boot), then you might be just seeing the disk swapping. look in About this Mac.

Download the free utility, EtreCheck. Use it to generate a report then copy and paste the report into your reply here.

I would follow the instruction of Kappy: execute the tool and post here the full report.

I had this problem as well after the 10.13.2 update. A PRAM reset fixed it.