Precision M4800 EXTREMELY slow

They probably didn't know anything about installing drivers.

I have bought plenty of used laptops on eBay, and many of the sellers are simply large-scale e-recycling companies. Don't expect much from them.

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When you have a situation like this, the best thing to do is initiate a return to the eBay seller.

I have bought a ton of refurbs and have never seen anything similar, other than a system with a totally dead HSF cooler. The i7-4810MQ is a powerful, quad-core running at 2.8GHz, and no matter what type of Windows 10 install or driver set, it should be running at that speed or close. This part is not rocket science.

If it's being artificially throttled to 0.35 GHz, then it could be a faulty component, a broken fan/cooling system, a problem with the HSF, or any number of issues, but you don't want to get involved in trouble-shooting, buying additional hardware, installing it all, etc. as it could easily be a dying motherboard, which is probably the same price as your entire refurb system.

Just return it and get another one.

Did you sort this out? Your problem is that you don't have a large enough power supply or  that your power supply isn't an official Dell.  Go into BIOS and disable speedstep and you should see performance come back up at the cost of battery life until you can get an appropriate power supply.

When system messages are suppressed in the BIOS and the connected power supply do not have the required 130W or more (as the biggest one for this Precision series), the system throttles constantly down (800 MHz on my side). When operated with the notebook battery only, the CPU frequency raises dynamically to it's maximum and the system works as fast as usual...

Mine was the power supply issue above thanks

@Ahoskin​ Thank you, I applied it and it was very helpful.