Connections for large splines

[DekoGames]

I'm using long Catmull Rom splines (500m - 800m)  to create rail track models and find that whenever I try to connect end CPs the performance slows down terribly (42 FPS down to 0.8 FPS). I've tried to segment the longer splines into short pieces with only 3 or 4 CPs but it helps only a little (2 FPS). I've also tried disabling Check Transform which doesn't make a great difference. If I use linear splines, however, the problem disappears.

Is there a best practice for using connections? Is it generally best to connect several short splines, or is it not recommended to use long spines?