Why is partition merging so slow? (Not specifically a Win10 question)

Hello,

I know this isn't a question that is specific to windows 10, it's just I'm facing it, again, while using windows 10, haha.

Simply: would someone know why it takes so goddamn long to merge together the partitions of a hard disk drive?

I tried to google for an explanation, but all I found were pleas for help after failed mergings, or companies trying to recommend their own brand for the operation, or else the wikipedia article for merging data in database situations - not what I'm seeking, thus.
But I must also reckon my google-fu sucks, I usually fail to find the proper wording that will yield good results.

I don't beg for a complete explanation right here right now, if you can find a link to a page with the explanation, that will be perfect :)

*

The reason I'm asking after all these years, if you wonder.

In my current case, merging a 955 GB partition with a 450 GB partition, my program (easeus partition master free) has been at it for over two hours, and from the looks of it I'm good for double that before it's done.
The disk usage (as noted by the windows tasks manager) has been between 60 and 80 MB/s since the beginning of the operation. It matches: at max disk speed, it would take around 4 hours to move the totality of the data in the largest partition.

So it's not a nightmare, it really seems the program has to move every single byte for some reason to proceed to a merging between partitions.
In my - clearly wrong - imagination, it should - no, it ought to - be done in a jiffy, just by updating the MFT, telling the system "look, you see that file? Now, you will find it here. Moving on. Look, you see that file? Now, you will find it here. And so on."
While, here, we're at max disk reading speed for hours, apparently data really has to be physically moved, isn't it insane?!? There must be a reason it can't be made in a faster and more efficient manner than that, and I'm extremly curious to learn it.

Thanks to whoever manages to find a link to an explanation, that will make my day :)

Welcome to the forum the maths behind this is herendous you have to calculate the files size start and stop then work out were its going to fit move it alter partition and repeat maybe 1,00 times

Samuria said:

Welcome to the forum the maths behind this is herendous you have to calculate the files size start and stop then work out were its going to fit move it alter partition and repeat maybe 1,00 times

Thanks, Samuria!

TBH I already suspected something filthy in terms of computering was going on, yeah. But would you have more info, precisely? Or even the proper keywords to find explanations in a search engine?

I don't pretend I'll come up with a better way of knowing, it's just I'm eager to at least understand why it's going so badly. A bit like a mechanic, I like to understand what's happening below the hood =)