Transition stutters

When my testimonials slide from one to the next, the transition is very very stuttery. I’ve tried playing with the relative speeds, but there seems to be no change.

Can anyone suggest any solutions?

The code I’m using is;

[sp_testimonials_slider design="design-1" slides_column="2" sp_testimonials_slider speed="4000" speed="8000"]

On my page here;

http://www.ckmartialhearts.com/

https://ww.wp.xz.cn/plugins/wp-testimonial-with-widget/