@Maximum-Dev You can try and do the same thing using Landscape Splines and see if you get similar results. Maybe it has something to do with how your landscape material was set up.
Spline becomes laggy to move around when you have too much road/extra components on it, it does not depend on how many points a spline has. I’m going to implement On/Off toggle for performance heavy computations and look into how things may be better optimized. As a workaround you can temporarily set Spline->XScale to a large value, greater value results in greater performance. Also disable all ‘Spline Extra’ stuff.
It is not recommended to have very long roads with many components anyway, if you do, you may consider merging them into static meshes as a final step.
Best Regards