Click see inside (don't open the "very laggy" sprite), then click file > save to your computer. The project.json file will be ~600kb. Now click the "very laggy" sprite and try saving it again. This time the project.json file will be ~6.6mb.
I found out a while ago that you can use the same block multiple times. This project has a single 500kb join block that is used in 10 move blocks. Because the same join block is used, the total project.json size is a little over 500kb. For whatever reason if you load the big sprite in the editor the join block unduplicates itself and there are now 10 500kb blocks, which makes the total project.json size over 6mb. The duplicate blocks seem to work as normal both before and after opening the sprite. In this project they don't do anything though. This could be used to compress the project.json even further, if you have a lot of duplicate reporters for some reason. It might also allow you to jump from any block to any block by modifying the "next" attribute. I haven't tested this though.
