Slow performance of Precision 7540 workstation

I bought the laptop for my work this March (2021). I was working on Linux 20.04 and all was just fine. One usual day (no installing a specific program or update) and after one month of use I noticed that the performance of CPU dropped by 4-5 times from the usual. I tried to load a dataset in jupyter notebook and instead of around 2:30'', it took around 12' and also all other small tasks taking more time to execute. Furthermore, it takes longer time to open programs and applications ( such as Slack, PyCharm, internet browsers). The fans don't operate at all in order to understand that the laptop executes heavy tasks. My CPU is Intel-Core I9-9980 at 2.4GHz with 16 cores.

I contacted with tech-support of Dell and they changed memory cards, motherboard, and hard disk but nothing changed. I installed Linux 19.04 and the speed was like before (very fast) but I had serious problems with updates of the software and I couldn't use this version. Then I installed Linux 21.04 and the same problem with CPU performance continues. Furthermore, the performance of CPU in Windows 10 was also fine (very fast).

I read the posts here and I downgraded BIOS from 1.11.2 to 1.9 but nothing changed. 

I need to work on 20.04 but I can't think why it has slow performance. I will appreciate any help.

A small notice is that at the beginning I connected the laptop with a Dell monitor 32'.