I've been struggling mentally with my ineptitude to change chords on time. It helped a lot when I've accepted it's okay and focused more on keeping the rythm with my strumming hand. Maintaining the rhythm makes slow chord switching much more bearable, so much so that even missing first half of a bar isn't a deal breaker anymore. And at that point,  just by having a relaxed mentality about being slow, I've found that my speed has improved. Hope this helps.