I think Select2 is still the best. I don't think we should move away from them- being able to type in them to filter is a great feature.
I don't think we have to get them to load faster to fix this. I think the most jarring part about it is the stuff around it jumping around. If we added some styles to the default dropdowns, so that when Selec2 loads, all it basically does is...display its gray down arrow on the right, then that would be very subtle and a big improvement.