I agree it basically doesn't matter, but there are all kinds of factors.
1) CPU Threading, if it is only written for one thread, then it doesn't matter if you have 16, going to use only one, so it comes down to the speed of a single core (or part of a core depending on how it is written).
2) Other software, Anti-malware, Anti-virus, Endpoint data protection, etc will all scan the installer as it is unpacked and run. Not the most common thing on a typical end user computer, but if you have Windows defender running, it is scanning all that stuff live. Not all of these tools accurately report CPU usage either. (In an attempt to hide themselves from malware usually)
3) Background processes. One big one is Windows restore points, backing up the registry. Sometimes this isn't obvious and might just show up as 'System'. Might not use a lot of CPU, but it will save a backup, and then verify it. Updating or merging .NET framework.
4) Software may have artificial delays added. I have seen this in some software, where they don't necessarily know how long something will take, so they overestimate to an extreme, put in a timer, and trigger the next step after that. Pretty rare these days with canned installers. Usually only saw it on things coming out of universities. (Oh the stories there, how they let some of this crap become business critical...well what do you know, the guy that wrote it graduated 10 years ago and doesn't offer support)
5) Most progress bars and percentage complete steps in Windows installers are 100% made up. So you can't really go with that. If the installer is waiting on something that they didn't code for, it will just wait.