Is it this particular asset in a particular area?
I recently stumbled upon what I believe to be the cause, not that the solution is a nice one.
The issue had nothing to do with the roads or other splines themselves, and much more to do with other scenery objects in the area. If there is one or more asset that's poorly constructed or has other issues, then it puts a drag on the game and causes the track and other spline assets to load up slowly.
On my large route where I put the two and two together, I simply removed the offending large asset, after a long process of hunting down the culprit. The fact that this occurred in one particular area was a clue. The offending asset turned out to be a large warehouse structure, a very old asset by the way, that was causing the problem. I've since replaced that with one of Dave Snow's better made buildings which looks better anyway.
Keep in mind that this is a solution to one instance of the problem and may or may not be the only cause. I recommend looking around your route.
Keep in mind too that some tracks and roads are made better than others, meaning some while very detailed are extremely polygon heavy to start with which doesn't help the cause. During my troubleshooting, I replaced my track with something else. This helped some but didn't completely solve the problem. The warehouse in this section of my very large route was culprit this time where the problem occurred.
