Check the network speed https://www.speedtest.net
At the end of month most of data storage is consumed , you might have taken plan from internet service provider , there are thousands of photos and it will take days to sync , if other data is also still syncing then also network speed will be slowed , either purchase a new plan or wait for next month in the beginning days of month the network speed is fast .
If these photos are were from different iPhones signed in with same Apple ID and password on the same network , syncing will slow using wifi .
The fastest method is connecting iPhone and Mac using usb cable see some articles About syncing your Mac and your devices - Apple Support
Sync content between your Mac and iPhone or iPad - Apple Support
Sync films between your Mac and iPhone or iPad - Apple Support