Hoping this helps someone deal with Tahoe problems if they're lost for solutions.
I'm usually quick to install OS updates, but Tahoe immediately caused major issues that I've taken 1-2 months to finally identify and solve. This includes: keychain issues, user permissions, and worst of all - major disturbances to overall system performance. Each issue became apparent quickly after updating.
My technical specs are a MacBook Pro 13" (2019) with an Intel processor and 32gb of RAM, I'm not sure if that's important or if this exists on Apple chips as well.
The biggest fix overall has been to system performance, which it seems was a combination of resetting my extended display settings for a new widescreen monitor, and a number of new default OS settings added by Tahoe as part of "glass" redesign. This amazing guide The Cleanest macOS Tahoe Install Guide (macOS 26) helped me identify every major setting affecting system power or hogging memory. While they at first seemed inconsequential, turning them off completely fixed most all random memory spikes and constant loud fan usage required to cool down the overheated system. I also have a pretty new widescreen monitor with native settings that got reset or changed and became a part of power hogging and other issues. It ran fine at first, but Tahoe seemed to reset my ideal configurations somehow.
If your MacBook went from zippy to slow or is running its fans all the time, even when barely any apps are open, it might actually be these new defaults set to "improve" the UI transparency but actually just causing more issues.
Tahoe also for some reason reset much of my saved keychain access settings for no apparent reason, and reset user permissions on many key apps, causing haywire. I eventually was able to solve this through a brand-new user admin profile and individually adjusting user permissions on programs installed in the wrong location. Not every program has this problem but a few constantly needed to ask permission for everything and adjusting these permissions or moving them to a better folder eventually stopped this distracting practice almost completely for me.
I've never seen Apple drop the ball and release an OS update this problematic, it's really disappointing to see. The "glass" design overhaul causes far more problems than it's worth, turning it off is barely even noticeable to me, visually. Hope this can help anyone whose been stumped, it hasn't been easy to pinpoint.
I'm not exactly a technical expert, but please let me know if these issues are familiar to you or if you have specific questions. Doesn't seem solutions are that readily available to all.