I do the same to load up the content quickly. When I start driving sessions, I pause prior to running so the trains aren't buried in the ground or running on no track. To me this looks like the program is reading in the small data-segments from disk. Having a quick disk will really help but not all of us have those. I notice my large hard disk grinds away until the content is loaded.
I have found that some tracks load better than others. The old JR track from TS12 days loads great, but their later track, such as the JR US series, really is awful with track disappearing and then loading in chunks when moving the camera or in a locomotive. I noticed that this track also affects other splines such as roads. My current track of choice is Trainboi's pro-track. This loads well and doesn't seem to affect other splines.
In addition to the track change, I also found that setting the VSYNC to half instead of 60 HZ or none really made a difference.
With these changes, I've been able to load up my large route without too many issues. This route is a combination of 5m and 10 m grid with some parts dating back to early January 2004.
When I converted the route to HD to test it, the file-size blew up from 350 MB to about 2 TB. I have Plus installed on a 12 TB hard disk, but with routes that size, that's not a whole lot of routes or room for anything else.