I would say one or more battery cells are going bad or something wrong with the charging circuit. Definitely nothing to do with the AC adapter.
Try resetting the EC chip. Turn it off, hold the power button until the LEDs flash, then power it on again.
It might be possible for a failing battery to cause this if it's causing the power mode to change. What happens if you just run games on battery without the AC adapter? It will perform worse but is it consistent or still speeding up and slowing down?
I buy a new AC Power charger with 19.5 V, 11.8 A and the same issue. The battery of my laptop not charging. Original AC Power charger has 19.5V and 9.23A.
The Windows show charging, but the percentage not increasing. The same... I have FRAMES DROPS in the games with the charger connected.
If I disconnecting the AC POWER adapter I can play the games without FRAMES DROPS on the battery only. At good resolution and set to HIGH or VERY HIGH. Now I have battery percentage 46% only... 😞
Today I try do disconnecting the battery amd I try to play the games with the charger only to see if the FRAMES DROPS persisting...
If is working fine with the charger only could be the battery? Right? Or if is not working fine with the AC Power charger only and the FRAMES DROPS persisting, the issue come from the new motherboard, changed in the warranty from the last 2 weeks ago...
I must send it again to the service to change the motherboard again 😞
The laptop is working fine, Windows, memory, M2, SSD, external devices, no problems. Only FRAMES DROPS in the games with the AC charger connected only 😞
CONCLUSION: I get good performance in games running on battery only, but bad performance while the charger is connected.