Hello,
I am experiencing the same issue! I actually could reproduce it in the lab without a mission. I was just taking geotaged images and QGC is displaying a camera icon on every place I took a picture. Once I get a certain amount of pictures, around 400, QGC starts to react slower. At some point it is impossible to control. If then restart QGC there is no problem anymore. If I repeat the operation, the problem occurs over and over.