rsync is fantastic for synchronizing remote sites and most often doing the compare without -c is sufficient. In this case progress is shown file by file because building and transmitting the list is pretty fast even for huge directory trees.
However, if one occasionally wants to verify all data by using parameter -c , building the list of all MD5 hashes on a big tree may take hours without any indication of progress.
I suggest that in this case rsync first determines the total count of files to be compared, and afterwards updates progress by showing "Calculating MD5 of file no. xxx of yyy total".
Once the list is built the usual display of messages can be resumed.