I had a similar issue on Windows 10 after upgrading from Arq 5 to Arq 7: backups that would finish in minutes on Arq 5 would take days to complete on Arq 7.
I tried completely reinstalling Arq 7 and starting fresh. Same problem. Arq support wasn’t willing to work to solve the problem (they said they couldn’t do anything unless I told them why it wasn’t working and showed them exactly how to recreate it). So I went back to Arq 5 and it reverted to the previous performance: backups complete in minutes.
The point is there is something different in Arq 7’s backup engine that results in atrocious performance in some cases.
My suspicion (which I did tell to Arq support!) is that it is due to Microsoft OneDrive’s use of “Files on Demand”, which is where the file system is manipulated to have a kind of placeholder entry for files that are in the OneDrive cloud but not currently on the computer. OneDrive is using NTFS reparse points for this, and it uses them even when the file actually is stored locally. I have some evidence that this fools backup programs into thinking that every file is changed even when they aren’t.
Do you have something like this on your Mac? OneDrive, or Apple’s Deskop/Documents redirection into iCloud, or something like this for Google Drive? Maybe Arq 7 has the same problem on macOS as it does on Windows.