The scanning time depends on many factors and among all the factors, the total file size and file types have the biggest impact.
If there are a lot of message attachments, third-party App attachments, it will make the scanning much slower.
You can try the following tips to speed up the scanning process.
1) Choose only the file types you need and scan the phone again.
2) If you have iTunes/iCloud backup, it’s recommended to try Recover from iTunes backup file and Recover from the iCloud backup file. It will be much faster in these two modes.