Hello from 2022!
Sooo.. Stumbled upon this thread through google, and I had exactly the same problem, though it had absolutely nothing to do with Houdini whatsoever.
Perhaps this will help some1 who is googling with similar issues...
Spline tool (any kind, either self made, plugin, houdini or anything) is being VERY SLOW. That is to say, you can't even move your mouse when trying to edit spline points.
In my case, this was due to fact, that my team is using EXTERNAL DDC.
So, when working with the spline BP, Unreal seems to create some small piece of cache. What UE would normally do - is store the cache on the local HDD. But since DDC is set to an external server, the bastard addresses that piece of cache EVERY FRAME, reading from the internet cable rather than from the hard drive! And there is our heavy lag!
So, all in all, the issues is solved by tricking UE to make a "local" cache and use it instead of the external DDC. I did that, by disabling my connection with our server, then opening the project and fiddling around with the BP. It seems to create the cache files it needs locally, and only after that, I re-enable my connection with our server for DDC.
Hope this helps some1.