What is wrong with the Clock app (on Sequoia first, even worse now on Tahoe)?

[Boneslasher]

Dear all,
I have been experiencing this since the latest Sequoia patches (15.4+) and now, in Tahoe (26.0.1), the Clock app is completely broken for me.
Back in Sequoia, I started to notice a distinctive slowdown in the Timer function only: clicking on the green button to start a timer would not respond until, about 20 seconds later, as many timers as you clicked would appear, all started at the correct time. Pressing on 'Cancel' would get the same behaviour. The button would not respond or, better, it would respond about 20 seconds later.
After updating to Tahoe over the weekend, the Clock app is now unusable: the 'Local Times' tab '+' button takes a few seconds to respond; the 'Stopwatch' tab apparently starts but there is no numbers on screen. Pressing the Lap button given a series of "0:00:00" laps; the 'Alarm' tab apparently works until, then, the alarm is not set; finally, the 'Timer' tab is just blank (or, well, black). No interface, nothing at all.
I have attached two videos below. Perhaps the Clock app is not something too used by people in general so few if anyone has ever reported this? Every macOS update or upgrade has been flawless, and the SSV would have repaired any damage during install if it found it or sent me to Recovery if it was so bad. Sadly, the Clock app is bundled with macOS so I cannot just uninstall it and restore it from the App Store.

Is anyone else seeing this too? Is there anything one can do? I had an Apple Assistant Feedback open in case you want to join the report: FB20663940.
Thanks!

 

[Boneslasher]

I would like to post here an update, after almost one month of intense testing, troubleshooting, calls with Apple Support and incredible help from generous users.

Introduction: every app that runs on the Mac has at least one .plist file (property list) in the user library (~Library/Preferences).

The Clock app has several and one of them was the culprit:
~/Library/Preferences/com.apple.mobiletimerd.plist

It seems that this plist, due to an obscure bug in the app, was not performing its regular housekeeping.
Over the almost 2 years of life of this M3 Max MacBook Pro, that plist grew from 1 KB in size and ~300 lines of code to 6.8 MB and 900.000 lines! This caused the timer process to crash because it was running out of memory (!). All this was proven by a careful analysis of the log extracted with the LogUI app.
I have now set a Reminder to check back with this plist in 4 months to see where it is.

The good thing is that my user account is in robust health, despite what all advisors wanted me to believe (according to them, I had to Erase the Mac and start fresh). Moving this plist to the desktop made macOS recreate it and the Clock app is now working perfectly!