This looks interesting. Will look into it when I have time to spare..
Amm.. Why do you use save files when there is adv dupe 1 and 2 ...?

Thanks!

Hmm, I have no such problem.. At least on the x64 branch of Gmod. What other options do you apply on the track pieces and do you use some other Gmod branch? May I have screenshot of all the flags used. It is right where "Ignore Physics Gun" is located. Possible cobinations tested are:
1. Freeze piece + Ignore physics gun: The piece is frozen and you cannot interact with it with the physgun so can't press reload or grab a pices to mess it up.
2. Freeze + Weld: The pieces are frozen and welded together. You can grab one with the physgun but you cannot manipulate it because it is welded to the other parts of the track. However you can press reload and mess the track up.

To reduce the lag to the minimum I recommend using these options at the same time:
1. "Freeze" ( Freeze as in disabling the movement of the tracks sawned )
2. "Ignore physics gun" ( You cannot mess up the track by hitting reload or grabbing )
3. "No Collide" ( Disables collisions and in the same time adds duplication support )

All three must be enabled for best UX.