Preview file size calculation takes too long

Have you installed and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Download and run Etrecheck. Be sure to give it Full Disk access before running.

Copy and paste the results into your reply. Etrecheck is a diagnostic tool that was developed by one of the most respected users here in the ASC and recommended by Apple Support  to provide a snapshot of the system and help identify the more obvious culprits that can adversely affect a Mac's performance.

Copy the report

and use the Additional Text button to paste the report in your reply.

Then we can evaluate the report to see if we can determine the cause of the problem.

Good morning!

The bad news is you bought the usually crippled "educational/institutional" model made for bulk buyers looking for a low price point. It has a slow, laptop-class 2.3Ghz two-core processor instead the regular iMac 4K 21.5" that has, at minimum, a proper desktop-class 3.0Ghz four-core processor.

The absolutely AMAZING good news is you had the wisdom and foresight to get it with the optional factory solid-state drive!!! That completely absolves all sin caused by the dual-core config for most users.

I have noticed the same issues re Preview recalculating file size. I've had them as long as Preview was a thing on multiple Macs, each one faster than the last. I currently have a built-to-order 2017 27-inch iMac 5K with the optional 4.2ghz i7 processor and a factory SSD only marginally faster than your SSD. Here is how my iMac benchmarks compared to yours:

and, guess what? In spite of s 2.5x raw advantage, Preview is still painfullyslow on that top-of-line model. I do not think it is anything wrong with your computer but that you are seeing rather a long-lived issue with Preview.

Your ordering the factory SSD? Well played!! 👍🏼

Boot into Safe Mode according to How to use safe mode on your Mac and you can check to see if you've removed all of the supporting files by downloading and running the shareware app Find Any File to search for any files with the application's or the developer's name in the file name.  For the two malware mentioned you'd do the following searches: 

1 - Name contains Smokyashan

2 - Name contains Sleepshadow

Any files that are found can be dragged from the search results window to the Desktop or Trash bin in the Dock for deletion.

FAF can search areas that Spotlight can't like invisible folders, system folders and packages.  

NOTE:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Please post the entire report in your next reply so we can evaluate it. There are things we look for that can affect performance. Follow my last reply as to how to copy and paste the report in your reply. Thanks.

The first thing you should do is start a backup strategy either with Time Machine or other backup app. Otherwise you'll lose your user data eventually.

Next you have Malware installed: Smokyashan and Sleepshadow. To get rid of them I recommend download and run the free version of Malwarebytes. It was developed by a long time contributor to these forums and a highly respected member of the computer security community.  Then rerun Etrecheck and see if either is listed in the report.

There's no reason to ever install and run any "cleaning", "optimizing", "speed-up", anti-virus or VPN apps on your Mac?

Glad to hear you got things cleaned out. Good job.

Oh, to be 77 again.