AWKWARD ZOMBIE

0 seconds sets it to your browser's default speed, so yeah setting it to that's going to be slower.

I should do a tutorial on this for Photoshop, since they're substantially different.

Bumpan with a relevant request.

How do I made gif from scratch?

You create each frame as a separate layer (the first frame being the lowest one, then set the duration for all of them in their name (ex. "Layer #16 (250ms)(replace|combine)" without quotation marks), than you optimize it if you feel like it, convert it from RGB to Indexed mode, and export it as a .gif with As Animation checked. You can watch the result without having to export it via Filters>Animation>Replay.