Demnus, How many components do you have on the character blueprint? We just found that on listen server side the only thing being interpolated correctly is the skeletal mesh. Any component you have attached will not be interpolated on the listen server screen. We were using a static mesh to represent the player character with a bunch of meshes for weapons and material billboards as attached components back when we were prototyping mach 3 speeds. we are still working through animations and probably will be for the next few months so our skeletal mesh is still being worked on but I just hooked up the skeletal mesh we had with out any animations and it looks smooth on listen side going 2500mph as long as we use only a skeletal mesh by itself. anything that I have attached though still looks like the 15-20 fps your talking about. This is why most devs ive seen are going dedicated on ue4 when it comes to high speed games. the listen server interp is just bad. I wonder if there is way to use the interp nodes to interp the location of all these attached components.
btw the network lag emulation works for now but we found a lot of stuff we had to rebuild the first time we tested dev builds in real world. we had a guy in new York and one in texas. I ran the server on my pc and my gf connected through lan. she was smooth and they were not. even with all the rpc we did. it turned out that it was better to use switch has authority straight to the multicast event and then remote would do rep to server which would then multicast. also when we ran with the interp provided by ue4 in the character movement component everything was buttery smooth for new York and texas but introduced the listen lag. I have searched far an wide for a solution to this listen server interp. the best I have came up with is interp on our lock on or straight up lock on to target which removes the jitter completely. not sure why this affected it all on the listen but it did. I think the camera rotation is updated after the client update so it makes it appear smooth because every frame is updating the rotation of the camera after the player positions are updated. ill attach a pic of the switch authority example that dude gave me back then. maybe it will help you with high speed as it did me.
These pics did not help with the particle system (which was broken because its activation would happen while it was not relevant). while they did not help with the particle system it did help a whole lot when applying the switch has authority to the mach 3 movement system.