The strength of the Gif format is that can hold animations in a rather simple format, playing instantly in any browser... and that's nearly all.
One of the major drawbacks, beside the fact it doesn't has a good compressor, is its limitation a palette of to 256 colors. A good animated Gif generator can adjust the palette for each frame (IIRC), but if you use the same palette for the whole animation, you are quite limited.
I think what you see is not a stroke, but indeed a bad conversion of anti-aliasing: going from millions of nuances to express the transition of red to white (or black), you have perhaps only a few dozens of colors. Perhaps there is some transparency information lost too (256 nuances to 2!).

Perhaps you can get a better result by making a series of PNG images and converting them to Gif with a good tool...
The Gif animation library is quite good, but can have limitations.

" slower"
Have you adjusted the delay parameter?

same question two threads?