Re: Dell XPS15 9650- Excel Slow ONLY on battery mode
This may not provide a solution, but may help you see what may be going wrong. Download Hardware Info and run it in sensors mode. Check the IA (CPU core) powers. Is it high and close to the Power limit PL1 threshold? If it is, it will cause power throttling and bring your CPU frequency down to its lowest level and PL1 will say Yes.
I have an Inspiron 5379 with an i7 8550u. PL1 is around 15 watts on battery and 23 watts on AC power. On battery power it has a high IA powers between 12-25 watts and there is always power limit throttling and it gets locked down to 399 MHz. Even on idle with only 1-5% CPU usage in the task manager, the IA power may be between 5-23 watts on battery power. On AC power, it may or may not power throttle, depending on how many tabs I have open with web browsing.
I sent it back to Dell to inspect, and online, it says they only replaced the hard drive. I know this will not help, since I have been running it with a SDD. I even installed Windows 10 on two other SSDs so I do not think it is a drive issue. Hopefully they fixed something else that they did not post online. I will check it out once it gets sent back to me.
Hopefully HW Info may give you some insight into what may be wrong. Good luck in your troubleshooting your laptop.