OK, so I've got a canvas with some sprites on it constantly switching costumes (associated blocks uploaded below) and it is lagging like crazy. I've read some other online forums and have somewhat self-diagnosed this to be caused by App Inventor not being great at switching costumes rapidly, but this answer seems wrong/doesn't exactly satisfy me.
(Canned Reply: ABG- Export & Upload .aia)
Export your .aia file and upload it here.
Manage you timer event well first.
How many images are you working with, what size are they ?
Here is a simple example, switching 6 images every 100ms
costumeChange.aia (45.2 KB)