It sure looks like the ACPI tables are suspect.
Looks like the latest BIOS from Dell is 1.10.1 and was released in Dec 2018. A BIOS update may fix things.
If it is an issue in the Intel PSTATE driver in Linux then a later kernel may fix things too.
Slowing down when plugged in is opposite of what I would expect.
Lets hope the kernel setting fixes the slowdown as BIOS updates have become a pain nowadays.
djb