Depending on the app/driver updating it can be cpu and memory intensive temporarily.
If you are talking about the hubitat platform, this is not likely the cause of your issues. Hub updates being available DO NOT affect your hub. Hell, there are people here still run 2.3.5 just because they don't feel like updating. It's one of the wonderful things about hubitat. They don't force you to upgrade and if you don't like an upgrade, it's incredibly easy to roll back.
Did you delete everything from the alexa side so it stops trying to reach out to your hub? (removing the alexa app alone from hubitat will not do this)
Honestly based on a lot of your complaints, and the fact you say it's happening to multiple hubs makes me thing there is something more fundamental going on. For diagnostics I would recommend the KISS principal.
All hubs on a flat network hard wired.
Switch wifi to 6 and below and zigbee to 20 and above ( make sure both hubs are on different zigbee channels. One of 20 and the other 25)
Make sure that jumbo frames are not enabled on any switch or router.
eliminate unnecessary switches
Make sure both hubs are set to AUTO in their networking section.
Now.... Once that is done.
If you are still having zigbee/z-wave devices issues...
Check for any z-wave ghosts. Eliminate if there are any.
Look closely at the logs (device, and event) for any anomalous behavior. This can give you clues to a rogue device causing mesh issues.
Install @thebearmay's Hub information driver as well as webcore graphs and monitor CPU and Memory activity. This could also give you clues to where issues are at.
Again, it seems something more is at work here that no one is seeing due to lack of some information.