That model is ten years old. The thermal compound has dried out and cannot adequately transfer heat away from the processor, and there may be a lot of dust inside the laptop and maybe inside the fan and heat fins. So, your laptop is unable to adequately cool itself. That will cause the processor to throttle (slow down). You might consider an internal cleaning. Open it up to clean out dust. Disassemble the fan to clean out dust and fluff. I found a fair bit in two laptops I worked on recently. Remove the heat sink, remove the old thermal compound and apply new thermal compound before reinstalling the heat sink. While you're in there, replace the CMOS battery. If it isn't dead, it soon will be. BIOS settings and date/time can't be saved without a functioning CMOS battery. 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.) Also, you might want to do what I did: I took a strip of lumber, about 3.5 cm. wide x 2 cm. high (1.5" x 3/4"}, and the length equal to the width of my laptop. I routed out two notches to accommodate the laptop's rear feet. The rear of the laptop sits in those notches, propping up the rear by 2 cm. That provides better ventilation underneath the laptop and makes it easier for the fan to draw in air via the bottom vent.  It's cheap insurance, and also positions the keyboard at a slight angle that makes typing more comfortable. It won't solve a major heat issue, but it helps somewhat.
My Dell Latitude E6440 gets quite hot on the left side of the keyboard,  and under the laptop. This is during normal usage with office work .    When I restarted it today after an update, I got this message : "Your system automatically powered off because it was warmer than expected. If this this problem recurs, please contact Dell and give this error code  #M1004
My Dell Latitude E6440 overheats