I'm having the same problem with my Latitude E6540 (Intel Core i7-4800MQ, 4GB RAM, 500GB HDD, 2GB AMD Radeon HD 8790M).

I have Dell Latitude E6540 and I get performance drop in every game (even with low requirements) after 3-4 months. Since that time I was trying many sollutions, but nothing seemed to been working. At first I was thinking this is software issue or windows 10 incopatibility. Reinstalling drivers, backing to older windows 10 builds - fail. Recently I found a page about problem with THIS Dell laptop model. Someone wrote that he fixed this issue by just opening "Windows Start" then "Power" and instead of selecting "Restart", click "Sleep". I done this and I was (and still I am) suprised. It fixed everything. I would never thought that Such a small thing could affect So much.

Write if it works for you too. 

PS: Sorry for bad English, I'm from Poland. Happy New Year!

Found it! Here you go: https://www.dell.com/community/Latitude/Help-Low-CPU-speed-problem-Dell-Latitude-e6540/td-p/7309586

Just go to the bottom. 

Well, the problem returned after upgrade to windows 10 1909. Again I tried some solutions. I found out if I make him run only on battery (I mean from powering up. Do not unplug after clicking power button!) the problem with performance magically disappears. So at first point I checked adapter, but no - it supply required power(130W). Then I go to power management and I checked out power plan configuration. I opened a power plan and then I navigated to "Advanced" -> "Processor Power Management" -> "Minimal Processor State". There is two Mode states: unplugged and plugged. The first one was set to 5%, the second one was set to 100%. I change them to 99% and (for "Holly Peace") in tab "Maximum Processor State" I set the same values.

So what was the source of performance drop? The Intel processor boost function known as "Intel TurboBoost" which only activates when the processor is working at 100%. "Intel TurboBoost" is nothing but "official overclocking" program which makes processor run faster (higher clock). Well, everything would be okey if that process would not need additional power. So from the power is taked from? Ofcourse from dedicated gpu card. According to that it only happen when processor reaches 100%, lowering this value to everything below (for example 99%) will be preventing ITB from running. Another fix is disabling this function in BIOS, but I didn't already tested this option. ITB could also be the major source of overheating. For many users it made their temperatures lower by even 30°C! So if you hit extremely high temperatures with your hardware it might fix it too. 

I really appreciate your answers bro (KonradK_2001). Thanks for your reply.

I've disabled my Intel Turbo Boost from BIOS and Installed Dell Feature Enhancement Pack. Then I've just set the power plan to Ultra Performance from Dell Power Manager and now my processor always stays at 2.7GHz.

Now my CSGO stays at 60fps even after playing 2-3 hours.

CSGO video settings I'm using:

Resolution 1280x720

Everything is set to low except Effect Detail it's on medium

VSync to Double Buffered

CPU Temps:

Maximum 83-85 °C

GPU Temps:

Maximum 80-83 °C

Thanks Bro.

Hey BRO Do I have to apply these steps every 10 minutes?

Plz reply...