Windows 10 lives its own life. Very unpredictable. "Process Explorer" (by Russinovich, easy to find on msft site) will help to understand what process is so greedy. I suppose it's something that maintenances windows.
BTW, how many RAM does your system have? Is it 32 or 64 bit windows? And also it's better a CPU has two ore more cores / threads.

P.S. Firefox is still getting updates even on Win7

Roman555 wrote on 2023-02-12, 16:31:

I suppose it's something that maintenances windows.

This, it can be as simple (and unwanted) as a virus/malicious software scan or update service that catalogs stuff. There are so many things running in the background at unpredictable moments. They usually don't affect much modern systems but are more than noticeable on outdated/unsupported systems, to the degree that they can bring the machine to an ususable state.

Obviously you really like Edge or you wouldn't go through the motions with it. But speaking for myself I had Firefox for many years which was a total hogger as time went by, then Chrome, but since I started using Opera I've never looked back, on desktop and mobile. Chrome users can even use all the extensions in Opera that they had before, integrated VPN by default, very light... If you'd like to try something new which feels familiar all the same give it a shot, I think you might like it a lot.

Roman555 wrote on 2023-02-12, 16:31:

"Process Explorer" (by Russinovich, easy to find on msft site) will help to understand what process is so greedy.

It would indeed be very interesting to see what files it's writing to. I would guess it's trying to self-update.
But it's also weird that it's causing such lag. How old is this "old machine"? Single-core?

What is your PC hard ware, list them please, photos would be helpful. My comments:

How much concern on the power usage? Remember, long use of old PC, less efficient and bit more power use, adds up to same amount of power watts usage as another more powerful, fast, efficient PC that is in low power use with normal use, hours of on (browsing etc) and bursts of heavy use (update, processing stuff (the one you are doing), takes a hour or less is equal watt usage.

A average newish PC 10 years old with PFC 90% efficient power supply doesn't even go over 100W for normal daily use, not gaming based around HP Z220. My configuration is like i7-3770 (not overclocked), 2 hard drives for data and 1 500GB SATA SSD for main window drive and a GTX 960 in low power mode.
So, keep in mind.

Also another PC at other end, small PC.
A Elitedesk 800 G3 mini fits the bill and is not to use more than 50W and uses i7-7700T 35W 4 core processor and can be had for about less than 200 usd on Ebay, most of them includes 8GB memory so ready to go. It has space for 1 2.5" SATA and 2280 NVME SSD. Pair of 16GB PC4-2400T sets you back about another 100 total or so (save up to get these later on). This can do jobs quickly on heavy stuff (updates and is done in a hour or less) and go to very low power when processing long tasks if need to.

Too old computer trying to use windows 10 Pro on it is wrong way to go even Core 2 duo E8600 is too old, if your PC is P4 or Pentium low end processor then very wrong way to go, because less efficient processor does uses more power over and takes longer time to finish, while i7-7700 at 65W or i7-7700T at 35W can do quickly and go to low power mode less than 15W in low power mode much quickly. The cut off for windows 10 pro is ivy bridge with i5 processor minimum and 500GB SSD minimum will save lot of power.

Windows 10 really prefers quick processor and is much efficient with fast processor and lot of ram. I upgraded from 16GB to 32GB stopped the lagging feeling. Windows 10 does not like 4GB at all ever. Even 8GB is borderline but 16GB is really the sweet spot.

Also, old PCs tend to be degraded with age and get more prone with crashes and corruptions especially if your PC is junk low grade generic PC which is more common in poor nations. Always invest in OEM quality parts always saves money in long run, i know this is costly but you can keep that PC for 10 years on that little HP Elitedesk 800 G3 mini PC with no repairs. Yes, it is true.

My PC I built, Z220 is now 6 or 7 years old started out as base, I purchased quality parts over the years as my savings allows. I have not seen a crash yet due to quality parts.

Cheers,