Dell Precision 5520 - Lag without any heat or performance issue.

That laptop is five years old, so I assume that you bought it used. Rather than trying to troubleshoot a system that may have non-optimal settings and possibly other issues, including malware, I would perform a clean install of Windows.

Install Windows and drivers

Use Microsoft Media Creation Tool to create the installer on a flash drive, boot from that flash drive and begin the installation process.

I wrote a detailed description of how to install Windows 10 and 11. It may help you.

Here's another description of the install process, from a Microsoft MVP.

Any drivers that are not provided by Windows Update can be found here: Support for Precision 5520 | Drivers & Downloads. After a clean install, the first thing to install is the Intel Chipset Device Software. I would install that chipset driver before connecting to the Internet and running Windows Update.

Monitor temperatures

It's also possible that the laptop is overheating due to accumulated dust inside and five-year-old thermal compound that is now drying out and becoming less effective. When a laptop overheats, the processor will slow down. You should install a free tool such as HWMonitor or CPU-Z to monitor temperatures of various components.

Clean out dust and apply fresh thermal compound

If it is overheating, it could be worthwhile to open the laptop to clean fan and exhaust radiator and apply fresh thermal compound. If you're not confident in following procedures in Dell's Owners Manual, you could have a professional technician do it.

Whenever touching components or working inside a computer, wear a grounded wrist strap, also called anti-static wrist strap, ESD wrist strap, or ground bracelet. It's a cheap and sensible precaution. Rest the laptop on an anti-static mat or at least a reasonable alternative such as corrugated cardboard. (Repeatedly touching a metal case part is not sufficient mitigation with modern components.)

Laptop is not overheating. I also cleaned it thoroughly. I don't know what is causing this.  

I checked thermals on throttlestop. They seem ok. Like 45 to 50 degrees. But it still throttles and stuck on .78ghz. I watched a video and disabled BD PROCHOT but it is not working for me very well.
I also read about unplugging and replugging your battery. I did that that today but no good results.
Can I have any solution to this. My job is on stake because of this dumb laptop. I did not knew that it is such a dull of a laptop. Help me.

When unplugging battery, did you press and hold power button for 30 seconds.

Also, Does it get CPU throttling when running battery power only?  If it does not, the issue could be with a bad connector or incorrect AC adapter.

Check system logs in BIOS settings for any recorded events that could provide more information about the issue.

I did press the power button for 30 seconds. The integrated gpu gets to 100 percent and then laptop starts to lag a lot. When I disabled the gpu driver and run it on the microsoft basic graphics then the cpu lags too.

dont know does this help or not but you should check your task manager in the gpu columm if there a "system" pop up and use the gpu 1 to copy then you just need to uninstall the current nvidia graphic driver then intsall an older version :D