Laptop's performance drops after 20 minutes of heavy CPU+GPU load, accompanied by the battery charge level jump from 100% to 94%.

After roughly 2.5 years of having successfully run the same CPU+GPU-heavy workload with no slowdowns for hours, my Precision 7560 has started slowing itself down after ~20 minutes. The picture is always the same:

1. In the beginning the CPU consumes ~50W, the GPU consumes ~90W, and the system in total ~180W.
2. In ~12 minutes, the battery jumps from 100% to 94% charge level at once.
3. In ~8 more minutes, the CPU drops to ~40W, the GPU to ~70W, and the system in total to ~150W. The rendered frame rate is at best at 80% of that at ~180W. The battery is at ~91.5% charge level, discharges.
4. In ~8 more minutes, the CPU drops to ~25W, the GPU remains at ~70W, and the system in total goes to ~135W. The rendered frame rate is at best at 50% of that at ~180W. The battery is at ~90% charge level, starts charging.

I'm using the stock AC power adapter rated for 180W.

Before all of this, I had updated the BIOS to the version 1.28.0 and noticed that my Long Life Cycle battery was not recognized as such anymore. Reconnecting the battery made the laptop recognize it correctly, but the performance drop remained. According to Dell Power Manager, the battery is in excellent condition,

however its wear level jumped from 8% to 16% in one month. The battery is 2.5 years old.

Dell replaced the AC adapter, the DC jack, and the motherboard, but not the battery that is still under warranty. The issue remained, using default BIOS settings does not help.

I got curious:

1. Does anyone know how to confirm that the laptop decides to switch the power supply from CPU+GPU to the battery in the spirit of Dell's Hybrid Power?
2. Does anyone know how to control that? I'd like to let the laptop discharge the battery to 50% charge level instead of halving the performance at 90% charge level.
3. What performance is to expect from the laptop running without a battery at all? I tried and got total system power consumption at below 110W with abysmal performance - and that with the stock AC power adapter rated for 180W.
4. Will using a more powerful Dell AC adapter help?

A laptop that can't use the total power output of its stock AC power adapter without its battery in new condition looks like planned obsolescence to me - and the subject here is one of top Dell's laptops.