if mod loaded, and go to the End dimension, look on Ender Dragon. the flight of the dragon becomes intermittent and slow

If I load MC with Forge + Forgelin + BP, create a new world, teleport to the end (/forge setdim @p 1) and look at the dragon, it acts normally.
There's probably something you're doing differently than I am.

ill try in depth testing:

i load clear mc-forge-forgelin-bp, create the new world, enter the End trough end portal frames setted in creative. also i try /forge setdim @p 1. - dragon freezes.
disable BetterPortals, load same world - dragon fly normal.

enable BetterPortals, load same world - dragon fly normal!
reload MC, create new world - dragon freezes.
dont disable BP - reload MC, load same world - dragon fly normal!
disable BP, create new world - dragon flies normal

so, like that