Slow responce from Sonar X3 producer

I have a project open in X3 producer with two instances of Kontakt 5, and some midi tracks driving an external sound unit. Every edit I try happens with a lag of a few seconds e.g moving clips in track view, or editing notes in prv. The delay is extremely frustrating and is rendering the program unusable. I have purged the samples in Kontakt to reduce the load in RAM, my drivers are all up to date,and the cpu  is not overloading.  Has anyone any idea why the program is behaving like this and what I can do to rectify it?  I have tried the cpu/graphics conservation trick by toggling the pause/break key, but this made matters worse. Thanks.

Thank-you for your reply. I opened up the project and before playback all edits were fine. After playback all edits were then again delayed by a few seconds - a kind of graphical latency. I couldn't find where to disconnect the synths so I muted them instead,and this temporarily solved the problem. However, after playback the problem returned , so I removed one instrument from one instance of Kontakt 5 and now things seem to be a bit smoother. Why this is I don't know.  I have not done any system tweaks, and I realize I'm reaching the limits of my system given  ever increasing sample sizes and complexity of soft synths, but I've never had this jitteriness with midi editing before.Here's hoping the problem has been resolved, or at least I know how to work around it. Thanks again.

Starting playback engages the audio engine if it isn't already which would explain why things slow down after playback. Try clicking the audio engine button in the Transport module to disengage the audio engine while editing if you don't need to hear MIDI notes auditioned. You might also try increasing audio buffer size in preferences to lower the CPU load, assuming you don't need low latency while editing.

Does your gfx card have a sound output?
If so try to disable in control panel hardware..

I'm at my daw now. The midi clips are all my own, recorded from my piano. I've examined every midi track (20)  and the only events listed are 99.9 % notes with the odd sustain controller on the piano track. In what I call my ' repair ' version with no synths loaded, I even deleted all the (suspected faulty ) imported midi clips, recorded some more ( albeit random  ) clips,and still Sonar decided not to play nicely.  That's with no audio playing and a completely different set of clips. The subsequent edits I try to make all take 5 to 30 seconds. It seems to get worse the longer the project is open. I have to confess I'm not looking forward to working on my next project as this problem came out of nowhere. I'm no expert but I have approximately 30 completed projects under my belt, and apart from the occasional crash, there have been no major calamities.  One last thought - would a Windows update cause Sonar to behave like this? My knowledge of how Windows and Sonar interact is weak, and I would appreciate an opinion on this. And one more last thought - thanks for taking the trouble to help.    P.s I disabled the sound driver on my graphics card, but unfortunately the problem still persists.