Just to bump and update this. It’s still a problem in 5.2 Preview 1.
I’ve submitted a bug report, but my guess is it will be a very long time before this issue is address, so time to start fiddling with temporary solutions. If I find one I will post it here.
I haven’t tested it yet but I’m sorry to hear that.
Honestly, I don’t understand why such an important question is still ignored.
Any update on this? This is making changing railroads impossible.
AFAIK still a problem.
I found a workaround, it’s not the best thing in the world but at least you can edit the splines without having major performance issues:
Hello, I compared the source from UE 5.0.3 with code from UE 5.1.0. There are differences that LandscapeSplines are now included in the source code SplineMeshComponent.cpp. I changed spline source SplineMeshComponent.h and SplineMeshComponent.cpp to the version from UE 5.0.3 and the splines are movable without slowing down. There is something with the LandscapeSpline that is now included, because the changes mentioned in some threads above are also included in UE5.0.3. Hope this helps.
Yes, this works a treat. I have had to do this for anything that uses the spline. Hopefully it gets resolved soon!
Thanks so much Masm! I was able to use use your workaround method and get it working somewhat. God I hope they fix this in 5.2. I tried it out in 5.0 and I’m still getting the slow down even there. 
Can you teach me how to do it? I’m new to this site.
Version 5.2 has been released.
Unfortunately it’s still quite slow to move the spline.
Nor have I seen any information that there will be a fix for this issue.
IMHO, this is the line that is causing the slowdown issue:
SplineMeshComponent.cpp(UEv5.03 ↔ UEv5.1+)
Basically, the IsCollisionEnabled() check has been removed from the RecreateCollision() function of SplineMeshComponent.cpp and now collision is always calculated even if I set my collision preset to ‘NoCollision’.
You can test this by removing simple collision on a static mesh that is used as a Spline Mesh and setting its collision complexity to ‘Use Simple Collision As Complex’. The spline performance will be great again.
This is still unfixed in the most recent file version on GitHub.
Here’s the fix that broke it: https://github.com/EpicGames/UnrealEngine/commit/1c9cd5b6c869ecb8e591cbab13e6d35044084d4c
I don’t understand why they removed the collison control of spline mesh. I tested it in UE5.2 by applying “Use Simple Collision As Complex” to the static mesh. There’s still a big performance difference between UE5.0 and UE5.2 when you build big rail.
However, moving the spline is %70 faster than before. It can be used that way, with some wasted time, at least until a fix is applied. Thanks for this information.
Bump, still a problem even with UE5.2 as far as I know. I do recommend using Castilla’s workaround though (earlier in this thread), it’s not ideal but it does work!
Yeah, we have a similar issue and recommend our clients to do the collision thing.
I ran into this problem and found an option inside class settings called run construction script on drag and toggling that option off helped me.
This should help any bp that creates objects from splines.
That worked! Can confirm this solution works in v5.2.1 
