A short clip often explains something better than a paragraph does. A GIF plays on its own, loops, and works almost anywhere you can post a picture.
A GIF has no video compression, so a long or wide one grows fast. Halving the width makes the file about a quarter of the size, which is the single most effective change.
Frames a second matter too. Ten is smooth enough for most clips and noticeably smaller than fifteen. Choosing the exact few seconds you need, rather than a generous margin either side, also helps.
A GIF cannot carry sound. If the sound matters, share the video instead, or save just the sound with the Video to MP3 tool.
Up to fifteen seconds. Longer clips become files too large to be useful.
A GIF is limited to 256 colours per frame. Footage with smooth gradients shows this most.
No. Every frame is read and built on your own device.