@BebeDahling I think I understand some of what you posted. I have the same MacBookPro as you, I think. It's a MBPr, mid-2015, 15" with 16GB of memory. This machine is very speedy and I don't see much delay on any website. You didn't say much about what you or your son have tried to diagnose the issue, but maybe we can help out some. I am running Catalina now, but I ran Mojave for a long time quite successfully.
What, exactly, do you mean by "speed?" Do you mean that when you are surfing the internet pages load slowly? Or that when you do anything using an application the system is sluggish? Those two issues are usually caused by two entirely separate causes or sets of causes. So we need a good description of what you mean by "speed."
You don't have the T2 chip, it is only in newer systems, so that's not the issue.
Well, getting XP to run on a Mac will certainly be an adventure. I doubt there is a good, easy way to do that. However, what kind of data are we talking about? Word documents? Excel spreadsheets? Photos? Most of those kinds of files can be moved easily, depending on how old they are and how far back they go in time. Old Word documents and Excel spreadsheets could be an issue, if the newer version of those products won't open them, but if those documents are even relatively current, Office should work, or one of the free Office-like products should work. Pictures should also be transferrable if they are a standard format like jpg or tif or png. Most of them are.
What Windows software? There may be a Mac equivalent, or replacement available. Windows applications will NOT run in any Mac OS at all.
Does that mean you have a 500GB drive with 168GB of data on it? Or 168GB free? You didn't say, exactly.
But before we start talking action, how about answering some of the questions, eh? running XP on the new desk top .