Better performance on battery than on AC

My laptop Fujitsu LIFEBOOK U9310 with Intel Core i5-10210U somehow performs better on battery, than on AC. Checked it with Cinebench R23 and got the difference of ~400 points in favor of battery. Undervolting didn’t affect the difference, it is the same, but with undervolting laptop performs a little bit better on charge than without undervolting on battery. And on battery with undervolting it gets around 2950 points in cinebench (on ac with undervolting - between 2500-2600 points). The results without undervolting are 500 points lower for both cases. The OS is clean, all drivers are from Fujitsu. During both tests the power management options in Windows were set to High Performance. In BIOS I tried disabling hardware power management, nothing changed. Also changing the behaviour of hardware power management in BIOS doesn’t change anything.
As far as I know, laptops should usually perform better on AC, than on battery, so the behaviour of my laptop seems strange to me.

Sounds to me like your laptop’s cooling is getting overwhelmed with the extra power used in whatever performance mode it enters while plugged in.

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Same thought here.

Looking at the pictures available online, it looks like cooling did not quite make the nice to have list.
Maybe prop up the back of the laptop (lego bricks, match boxes, etc.) to see if that helps.

Changed to Cinebench R20, so it will be easier for me to compare with other similar laptops (other laptops were benchmarked by R20). Tried reinstalling fujitsu’s power settings on Windows. Now there is no difference with the “Best performance” mode, that can be set using the taskbar battery icon. Another thing I noticed: after turbo boost it throttles to 15W power limit (Even though the PL1 limit set by Fujitsu is 18W). At this time CPU’s temperature is around 85-83°C. Propping up the back didn’t change the situation, by the way.

Is your laptop old or new? A good cleaning and application of fresh thermal paste is a good starting point.

The thermal paste was changed about a week ago.