Please help - incredibly slow (new) computer

First, let's start with the fact that I know nothing about computers despite having used them for 30 years for basic stuff like Word/Excel or browsing the internet.  I'm at my wits' end with my home setup - I got rid of a 10-yr old HP desktop because it was so slow, taking 10 minutes to boot up and 30-60 seconds to open webpages. I thought it was because the computer was so old.  Last fall I replaced it with a new Dell Inspiron desktop with Intel Core i3 CPU, 932GB storage, 8GB RAM, running Windows 11 Home. 

I loaded up all my old files on to the new computer, files and programs which consists of almost nothing but photos (more than 30,000 of them, about 100 GB), plus my old iTunes library, which I still use with my ancient but still working iPods.  Plus a program called HAP which allowed me to load a few thousand CDs worth of music onto an external hard drive Sony player.  That's it - a total of about 250GB of data on this brand-new 900GB machine.  No gaming, no performance-hogging programs of any kind, not even Word or Excel on this computer.  

Yet as soon as I copied it all onto the Dell, the Dell started behaving just like the old HP - taking literally 10-15 minutes to boot up (especially if I haven't turned on the computer in a few weeks).  When I click on the Edge browser shortcut, it might take 30-60 seconds to open a webpage.  To switch to another website might take 10-25 seconds.  It is intolerable - like the early days of dialup internet.

So I called Dell and they went through a number of diagnostics and said there was absolutely nothing wrong with the computer or its performance, and it had to be something with the Windows operating system.  But it's impossible to reach Microsoft and they'll just direct me to these forums anyway.  I wondered if it was somehow related to Windows Updates or Norton Antivirus.  A friend said I could delete Norton because Defender is just as good, so I did that, but that did not improve the performance.  I have all the automatic updates turned on, so it's not like Windows is way out of date.  Now I wonder if maybe it's the 20-year-old iTunes software or other legacy crap that I transferred from the old computer that is clogging up the system, but I have no way of figuring it out.  I'm happy to pay somebody to dig into all the systems and hidden files to figure out what is slowing things down and fix it, but I can't find anyone to do that who I'm confident knows Windows 11 cold and is a legitimate problem-solver who isn't just going to rip me off.    

Does anyone have any idea how I can find somebody who can get to the bottom of this, someone reputable who can remotely diagnose the problem and figure out why it is happening?  There's no way I can figure this out on my own, as I don't understand anything about software or even know what programs may be running other than those I mentioned.  I've pretty much stopped even using this new computer because it is so hopeless, and all it does is sit there storing my photos and iTunes (which is impossible to use because it's so slow).

Any ideas appreciated, but I need someone who can dive into the computer and find out why this happened.  Thanks for any advice.