Hi Randy, thank you for sharing your experience!
We have had the same and other issues. Luckily there has been improvement (for us at least) on latency, and recently there was a fix to the subtask issue.
This team came from using a different board product, so a board is a core part of their standard workflow. Since current issues are not disrupting work so we are going to try deleting all the archived cards and see if that improves performance.
I just posted an idea for auto-deletion of cards if anyone would like to vote!
Boards - function to delete cards via board settin... - Adobe Experience League Community - 705537