255% available (plugged in)

@random-name are you sure it worked fine before the november 2019 update? I'm having this problem since either 1709 or 1803.

And where are you getting the 34% performance number? You can force windows to run in performance mode with or without a battery.

I can't pinpoint it directly to the Nov 2019 update, but I am relatively sure since, I use my laptop >90% of the time on cable without battery inserted and I didn't have this problem in times prior to the update and did not have any other major changes. However, you may be right: I noticed the reduction in performance after the Nov Update, but I can't be sure that the 255% were not shown before... I get the (about) 34% number from the task manager CPU utilization. And yes, I set my windows to run in full performance mode with or without a battery, but the issue remains until I insert the battery. So, I am not sure if the 255% issue is actually causing my main problem, but it seems related.

Sorry, I know all this sounds strange, confusing and vague. I just wrote this here mainly to see if someone else has this strange issue.

Setting the computer to full performance doesn't do anything if you don't have that power plan in windows power options.

This is how it should look:

This is how mine looks:

As you can see i only have the balanced profile. Changing the laptop to high performance on the little battery icon on the lower right corner doesn't do anything.

I had to manually alter the balanced profile so it works at 100% in the processor power management:

Ok scratch my last post, it seems that even with just the balanced profile, changing to "high performance" in the battery icon on the lower right corner does work.

If i chose powersaving mode, my cpu doesn't go above ~700Mhz

Choosing balanced, the cpu only goes to its maximum when there is system load

Choosing high performance will put the cpu at its maximum frequency even when idle.

Thanks for your efforts, Miguel Simoes_PT. Actually, I had tried different Power Plans and settings earlier to force my tasks being able to utilize the full CPU - all without success. However, triggered by your post, I switched back from my "High Performance" Plan to "Balanced" and moving the slider at the battery icon to "Best Performance". Believe it or not, this actually solved my problem! Now, even without my battery inserted, I can again use 100% of my CPU... What a confusion... Thanks for your reply :) 

For all others reading this: Not sure now if my performance issue is at all related to the original problem of the 255% - I initially thought it is because my battery capacity would be 'outside' anything that my performance plans can capture. But now I still have the 255% shown, but performance is ok.

Sorry for the confusion.