Simple Multiplayer FPS Lag (LAN)

Hello, I am working on a multiplayer first person shooter and I have a problem where there is lag in the players' movements from each screen when I run the build and editor.

I am trying to go through the multiplayer FPS tutorial on Youtube by Brackeys, but it appears laggy. When I tried to translate this to a different tutorial, it worked great strangely, but I can't modify anything or it goes bad.

The lag looks like it is gittering/unsmoothed - movements seem to take around 0.2 seconds longer than it should to take effect - and it looked a bit like the send rate was really low. One player was often worse than the others (I think the editor is always better).

I do not believe it is a problem with interpolation or the network send rate as I have changed those (saved and ctrl-B), and found no effect (on anything at all).

Things tried:

• I tried to fix the problem by updating unity to 5.5, but I didn't notice any difference.

• When I translated what I have done, with the exact same player prefab and stuff, into a different multiplayer tutorial, (the Udemy course 'ahoy matey' from 'Learn to Code by Making Games'), it worked perfectly, however when I made a copy of this and tried to modify it, (so that the Hierarchy, assets, etc looked somewhat like the original Brackeys tutorial) it started lagging again, not totally badly, but still not smoothly and fast.

• I tried swapping around the custom and in-built network managers (from udemy and brackeys respectively) and found no change.

• I tried changing the send rate from edit-network settings and the player network transfrom, but didn't notice anything (at all).

• I tried redoing almost all of the Brackeys tutorial (except movement and setup scripts for the player), but its very laggy now.

...

I'm going mad with this so please help if you can think of a solution...!