iMac High Sierra slow progress bar during boot in 2013 iMac

I've got a Late 2013 27" iMac, pretty much maxed out - 32 GB ram, 1TB SSD, and the fastest processor available at the time, a quad core i7.

It worked well for quite a while. But sometime last year, it developed a couple of problems.

I had it originally (I think) on El Capitan, then upgraded to High Sierra about 18 months ago. About 6 months ago, it developed the problem. About three months ago, it got another symptom.

Problem 1 is that during boot, it takes a looooonng time to get past the white apple progress bar. I mean, like 20 minutes sitting at 80 percent. This happens at every boot. It also happens if I try to boot into safe mode.

Second symptom is that, when screen sharing with this mac, the screen sharing performance is terrible. Mouse clicks and keyboard activity are very delayed. If I select some files and try to drag them, sometimes it misses the select and ends up registering the click somewhere else.

I've tried the screen sharing with another client, Chicken for the Mac, even worse performance. I frequently screen share with 5 or so other Macs both local and over a VPN, with very good performance, just not the iMac anymore.

I've tried:

• Resetting the SMC
• Resetting NVRAM
• Rebooting into Safe Mode and then normally booting
• Re-installing High Sierra through the recovery console (i.e. not wiping the disk, just an OS reinstall).
• I've also turned off any startup items that looked dicey or old.
• I've unplugged all peripherals
• There is plenty of free space on the boot drive
• I've run first aid on the startup disk and there are no issues
• Verified that TRIM is still enabled on the startup SSD

I'd like to update to Mojave, but want to fix this issue first if I can.

Anyone have any ideas or experience?

Thanks