Change a GIF's speed, direction or frames

You can ask, but the format stops being honoured. GIF stores a frame delay in hundredths of a second, and below roughly 20 milliseconds per frame most browsers ignore the stated delay and substitute 100 milliseconds - so an animation asked to run ten times faster actually plays slower than the original. This page caps the delay at 20 milliseconds and tells you when it has done so, rather than writing a number your browser will throw away.

Speed changes how long each frame is shown and keeps every frame, so the motion stays smooth and the file size barely moves. Dropping frames removes some of them and gives their time to the frames that remain, so the running time is unchanged, the motion becomes choppier, and the file gets smaller - often much smaller. They are different requests, which is why they are different controls here.

Each frame keeps its own delay, so a final frame that held for a second still holds for a second once it becomes the first. Reversing only the order while leaving the timing in place produces a noticeably different animation from the one you asked for, and it is a common bug in tools that do this.

Two reasons. Playing out and back does not repeat the first and last frames, because a ping-pong that shows the turning frame twice stutters at the turn. And the count reported after rebuilding is read back from the file that was written, not from what was requested - GIF rounds every delay to a hundredth of a second, so the numbers in the file are not always exactly the numbers asked for.

No. It is decoded, rearranged and re-encoded in your browser. Reading the frames needs a decoder that Safari does not provide, so this page works in Chrome, Edge and Firefox and says so rather than failing quietly in Safari.