Laptop running hot

[Phil]

Kubuntu 20.10 running on a ThinkPad T420 with 6GB ram and a 500GB SSD.

I'm currently in an area with poor mobile phone reception. An Internet 
connection is via a tethered mobile phone to a USB port on the laptop. 
The download rate is in the 100s of bits / second with an occasional 
jump to 1 or 2 kilo bits per second. This may not have any bearing on 
the current heat problem, however, it's only occurred since I've arrived 
here. The heat problem is extreme if I attempt to download something 
from YouTube.

I did have about 100 web Firefox pages open and I have closed those. 
There aren't any applications running in the background that I'm aware 
of apart from a dozen minimised Dolphin directories. Top shows 134MB of 
free ram (which seems to be lower than expected)  and that the 2GB of 
swap ram is not being used.

I think I've ruled out a hardware fault because the laptop also runs 
Windows 10 and Internet connections seem to be a little quicker and the 
laptop doesn't seem to run as hot.

All suggestions will be appreciated.

-- 
Regards,
Phil