Hi guys, thanks for the quick replies!
@jonathan: yes, codepen does seem to add some extra lag/stutters. I am not really bothered by slight stutters of 300th of a second though. It's in firefox where it really starts to get problematic. Did you check in Firefox? Here is an example outside codepen for better reference: http://preview.frodoschering.nl/pattern-animation-2/
@jack: I tried a png with the same results. The threshold you are referring too sounds like a possible cause of the problem although I am not sure how I could fix that. I did not try using a regular img element instead of a background-image yet. I will try and see if that helps.
I am also wondering if I could do something to change/simplify the animation somehow and improve performance?
Thanks again for the help, really appreciate it!