Hi and thank you for welcoming me. I have made a completely clean KSP install and still experienced the issue only while running Trajectories. Today I installed FAR 1.3 and found that the lag disappears as soon as I disabled "Show Trajectory" in the Trajectories in game menu. Needless to say the lag also disappears entirely if I run KSP without Trajectories at all. It must be something in my configuration given that you are able to use the two without detectable issues. Perhaps my experience may help the other user who posted about the same lag I experienced while pitching before disabling Trajectories.
I experienced this same issue using the Dev build this past week. I have yet to use the new 1.3 release, but I will tell you what caused the same lag for me - especially while pitching - the "Trajectories" mod. I isolated the problem down to Trajectories (my other 40 mods work fine with FAR without Trajectories). I removed the Trajectories mod and FAR works flawlessly. I will go test FAR for 1.3 this evening and will come back to post should the issue persist without Trajectories. Hopefully this is an appropriate place to share my experience with this issue (first post here).