Alright, it sounds like maybe you don’t know about all the benefits of ECC memory, so I’ll just explain why I think it’s a good idea. It might be the stuff you already know, but might be new to some people.
One of the biggest dangers of non-ECC memory is that it can randomly bit-flip, without any warning whatsoever, without causing errors or crashes. If the bitflip (changing a random 1 into 0, or vice-versa) happens somewhere in the program code, or the kernel - that may lead to a crash. But often it does not, because the bitflip happens in the data. Now the danger of this is that your data can get corrupted, without you even knowing about it for a long time! One bit might not sound like much, bit if this happens in conjunction with encryption or archiving, one bit might just be enough to make the whole file or archive unreadable. If you don’t know about it, you might store this file on a hard drive for a long time, longer than what your backups cover, until you realize that the file is corrupted and you don’t have older copies of it.
So in this case, even if some systems show the slowing down behavior - (even though I’ve never experienced this due to ECC) - it’s a much much better option than silently corrupting your data. If the slowdown happens, you can just save all the data and restart. Whereas without ECC you wouldn’t know the data was corrupted, or you would just get a crash.
Such bitflips in regular memory is not a hypothetical scenario that happens once in a lifetime of a universe, this is a common occurence. "There is roughly 3% chance of error in RAM per DIMM per year."
There was even a case some time ago of someone hacking google by registering domain names that would get used by google servers due to bitflip errors. For example Google would call “cdn(dot)google(dot)com”, and then sometimes there would be (very specific, so extremely rare) bitflip that would change the string in the google server from “cdn(dot)google(dot)com” to “cdb(dot)google(dot)com”, due to a bitflip - and that server would instead try to load data from that bogus server. The hackers have registered a bunch of these domain names based on all bitflip combinations, and actually started receiving a great deal of internal Google traffic. (This was some time ago, when strangely enough Google did not use ECC on all it’s servers.)
The point is, this is a real problem.
If your PC is only used for gaming, movies, and some schoolwork, then paying extra for ECC probably doesn’t make sense. But if you do any important work, it just seems extremely stupid not to pay the little extra and know that your data will be more safe.
On a related note, I do see how Xeon’s power consumption (and heat generation profile) is an issue, but perhaps they could simply underclock them? Xeons are usually more powerful than mobile chips, so underclocking them would still provide a powerful enough alternative, without pushing the heat emission envelope too high.