I fixed it for myself. When the mod authors release their new updated, I'll slap the Charizard edit into it.
A couple things I changed:
1. Charizard sprite has been optimized better since most complaints were from handsets rather than computer. There is no noticable difference in the new modified sprite compared to the original.
2. Gengar's shiny coloring is now more ghostly white, however, this causes him to deviate from the way it was intended. I did that so that I could see it.
2.1 Sidenote: I also changed the follower mod sprite accordingly.
I did these changes for myself.
I should say that the stuttering people encounter has to do with the RAM on their phone. Even flagship phones like the Note9, Note10, A50 5G, A71 5G do stutter initially per session. Without really optimizing, I few the quality will also be lost.