Unless you are specifically playing the animation via the network (not sure why you would ever do that), then they are played locally. This could probably just the significance system, which has been in since the early days of Paragon. As objects become less significant as Epic puts it, they reduce the animation playback rate for better performance.
Here is the documentation:
As for the jittering - that is most likely caused by network smoothing (assuming the high speed movement is running through the normal character movement system and not by just setting the actors location). When characters move at high speed it’s more likely that the Server and Client will disagree on movement results (especially when they are running on different machines) and the client will be sent an adjustment. Try increasing the Max and No smoothing distance in the character movement component, and use the console commands to visualize the updates, such as “p.NetShowCorrections”