@sgillen thank you! Unfortunately we don’t have the best solution for now 
rosbags takes a long time to cycle through all the messages, and I do wonder if there’s something that could be done to speed that up.
A couple minor built-in things that make some small improvements in load time are (1) not converting msgs to images until a specific image is desired. Often, I don’t use every single image, so this prevents unnecessarily converting images aren’t used, and (2) if you know a specific time range, msgs outside of that time range are not loaded. Your question actually made me look into this a bit more and I think I should be using rosbags AnyReader’s start and stop params to maybe even get better performance - I’m doing this within robotdatapy right now, but I wonder if rosbags could use this to filter/speed things up further upstream.