Once or twice I've had 1GB per 30secs to ext USB2 HDD.
Usually, 1GB/min or a lttle over.
I always use Max compression in Advanced Settings.
Overall, speed will depend on content and how compressed it is to start with and also if you have lots of small files compared to big files.
There is a Verify option for each Image you have made. Click the Restore Tab (Top left-between Disk Image and Log tabs)
Personally, I just prefer to browse to where the image is saved and double-click to Mount it.
Use File Explorer as normal to browse the Mounted drive and copy any files if you want instead of making a full Restore.
You should get an error or it will be obvious if something is wrong with the Image file.