Hey there!
So the way that Parkour works is it based on 1.8.9. Every version of Minecraft has different ways of movement. Currently, it is in 1.8 and until it is updated to a higher version lag back/rubberbanding will occur. The answer to all your problems with lag back/rubberbanding is use 1.8. Hope this helped you understand!!
Beast Regards,
Ickysix