I do also often use grouping with a similar usecase than described at the beginning:
As soon as you use grouping not every item needs to be compared with all others but only each item in group e.g. 1 needs to be compared with group 2.
=> it should be linear, isn't it?
A further improvement might be: as soon one match is found (often that meand there is one song already double) further comparing could be stopped for that items as it is not neccessary to know that there are more than one duplicates...
Also I notice that count goes up to number itema Group 1+2. Group 2 will be the one that can be deleted with automarked files than the count should only go up to number items of group 2?!
Best, Fred