Larger animations (in pixel size) do have lag. It has been a known issue for a while now, however smaller animations do not have the same result. I have done a great deal of testing on these, and I have found that anything over about 12x12 units or so (1680x1680 in pixel size) do result in a "stutter" at the looping point of the animation. Unfortunately weather animations are going to be larger in size and will result in this behavior. It's not even a matter of file size as most of the WebM files I have uploaded are under 2mb, but seems to be related to the actual pixel size. Smaller items (like torches and trap tokens) don't get this stutter, at least not in my extensive testing.