This happened my twice. I had the same issue in 2018, after 7 months I had bought my XPS 9560. Support couldn't find any error, so they changed the fans, motherboard and charger.
But this was a bad solution because the problem reoccur after 6 months I had started using Photoshop and After Effects. So they came again and did the same routine. Silly me I did not demand a change or renewal.
Now after 6 months it is the same. I did everything with Windows support, a clean install. But computer is still lagging and CPU is stuck at 0.78 GHz. Now my warranty is finished, I don't know what to do.
In some cases, this gets fixed after disconnecting the battery for a minute (you must open the bottom panel following the online service manual).
If it doesn't help, you can try debugging throttling causes using HWinfo32/Sensors or Intel XTU.
I have the same issue, I did not find a solution until now...
The only guess I have is that it comes to protect the computer for some reason or it is a design limitation after some time of use of computer as it seems to be an old unsolved issue on the dell users community...
I have a very similar issue, mine (XPS15 9560 i7 32GB 1TB) can't ever get above 1.89GHz but often gets stuck at 0.78GHz and then needs to battery to be disconnected to re-set it.
It seems to be (at least in part) a power throttling problem, (which also seems to be depressingly common on these models), but the problem is much worse since the recent updates - although hard to know which specific update might have caused it.
It happened to me before, after an update a few months ago. I solved the problem then by doing a system restore back to how it was before the update. I would prefer a solution that does not involve never updating my computer again!
Monitoring using Intel Ultra Tuning app shows power throttling active when it is stuck at 0.78GHz.
It seems to get triggered when the Intel GPU is doing a lot of work (the NVIDA GPU hardly seems to be active at all). Just watching YouTube full screen is enough to cause it.
There are several online fixes involving re-applying thermal grease and adding thermal pads to the voltage regulators, undervolting the CPU etc. but this must be partly a software issue as the DELL updates seem to be creating the problem (at least for me).
Following the link in the original post, I opened "Dell Command Power Manager" and found that, in thermal management, my system was set to "Quiet". Changing it to "Optimised" or "Ultra performance" has removed the 1.88GHz cap on the CPU speed.