HD3D I don't have any production metrics since I'm not using the extension myself. The statement is based on how it's implemented and because I never heard of anyone having issues previously.
I think the most important part is how many users are included in the visible readers. This was designed as a moderation tool for the original sponsor, intended only to list a few groups with not too many users. This is the warning that's in the README file.
If there are hundreds or thousands of visible users, I definitely expect issues with the current implementation. But when the number of readers is just a few dozens, I don't expect any dramatic performance decrease since it's just one more standard relationship.
I would expect the performance to be similar to the Likes extension if every user liked every post they saw.