As well as all my other fixes I'd tested, I'd forgotten to mention this step which I applied today that helped immensely:
I followed the recommended procedure of restoring a timeshift snapshot from the grub boot menu a day ago, I then did a GUI timeshift restore of my last snapshot once I booted into my desktop (as recommended). Afterwards shutdown and startup took much longer, and my system was extremely sluggish. Today I repeated what I thought may have helped with this type of system slowdown once before.
I opened the timeshift GUI and deleted all my backup snapshots. Some snapshots required being deleted twice before they are removed from the menu. All my snapshots are manually created (not autosnapshots). Quotas are disabled in the system and in timeshift. After all snapshots were removed, I then performed a btrsf balancing. I performed the balancing twice, the first time will take a fair while depending on how long its been since the last time. The second balance (although redundant) I simply run to be extra sure everything is in order. This is likely an excessive use of balancing, and probably is not recommended nor required.
After the balancing is complete I reboot, and the change is noticeable immediately. The system shuts down in the normal amount of time, and the reboot time is normal as well. As soon as the system is fully started a series of tests proved the system performance was back as well. I have performed this sequence several times now and in both cases it has definitely improved things considerably. The first time I performed this along with many other troubleshooting steps, so I really wasn't sure what improved matters.
In this case I'm fairly confident this was the key step in correcting the performance issues. I can't say if this will be a help for others as I'm using "on demand" snapshots with quotas disabled (not the standard timeshift default setup). The thing that led me to this was reading numerous posts that an excessive amount of snapshots could lead to system slowdowns. In my case I only had 4-6 snapshots, (hardly what I'd consider excessive). However, the difference was like night and day after the deletions and re-balancing steps.
Perhaps that may help some.