sounds like "Flickr Pro members get ad-free browsing for themselves and their visitors" needs to change.
Certainly you could make it in-obstrusive - just have a non-moving, small one in the header and a a larger in the footer.
Don't have any ads next to the photo.
Kicking the 'adult content' off the platform was the first step in that. This 'we tried to beg for money' letter is a good second step.
Hopefully your AWS infrastructure serves the upcoming ads faster as well.
I was never aware that speed was an issue with flickr. When a place has original content that is not available elsewhere, there is no competition for speed, imho, ymmv.
I can't buy a pro account, my content which was public on flickr for years, became against the TOS when smug bought it.
There were several marginalized community groups there that never transitioned as a whole either.
Sad, it could of become the social network better than fbook.