iPhone 15 photos slow to import to Mac Studio via USB-C

iPhone 15 still uses USB2 while new Pro models use faster USB3.

BUT that does not explain your very slow wired transfers. Is /Applications/Image Capture.app as slow as Photos.app? Have you tried another cable? Where is the Photos library stored?

Do you use any 3rd party "cleaners" or virus apps that might intervene? An EtreCheck report would eliminate much guesswork:

Using EtreCheck to Troubleshoot Potential… - Apple Community

Bradley Rex wrote:What is this delay?, and how can I reduce it? It can add 30 minutes to doing something simple like an eBay listing, or photographing documents.

That sounds pretty awful. You list a MacAir-- does that work better? You don't say what macOS your Studio uses…

Where is your Photos Library on the Studio? Is it in the Pictures folder on our Mac's internal drive, or is it on an external drive? If on an external drive, how is it formatted? To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

If this drive is in a an incompatible format, stop running Photos with it immediately!  A Photos Library can sit on an incompatible drive, but running it may corrupt the database. This is so important, that I had to throw it up first.

If the format is OK, then to narrow things down, try these steps:

• Restart the computer (of course)
• Re-start in Safe Mode. This bypasses certain potentially disruptive processes, and it often helps. Safe Mode is different for different computers, so see this: 

           Start up your Mac in safe mode - Apple Support 

• Use another library if you have one, or make a small test Library with a few pictures in it and see if the same thing happens. You can create a new Library by closing Photos and then option-clicking on the Photos icon.
• Make a new user and see if the same problem recurs with it.  This is a bit harder, because a new user can't access a different user's stuff. You would need to temporarily move your Library out of your own user's Pictures folder up to the general "Users" folder. 
• Rebuild your Photos Library--close Photos and re-launch by option-command-clicking the app icon. If you use iCloud Photos, it’s possible that this will cause a re-sync that lasts long enough to make you nervous.

The idea is to find out if the problem is with the Library, with your account, or with caches and login items, and stuff like that. One of these steps may fix the problem, but their failure to work also helps figure out what's happening.

Let us know what happens…