Andy summed it up perfectly. As route builders we can go hog crazy putting in all kinds of details, some even many miles from the camera. The route will run perfect on our own machines then we'll get the idea of sharing our creation only to have the other users complain about the slide show. The thing is you need to plan what you are going to see and what is really necessary for the scene. Believe me, I've learned this the hard way too. Recently, I've started a new route which has nothing except the topo-map image on it where no one will see the landscape from the train or trackside. This planning, viewing, rechecking, and placing objects has actually been a lot of hard work. In many ways it's a lot easier to splatter down a bunch of trees, buildings and grass all over the place, within reason of course, and be done with an area then move on to the next.
As has been mentioned, there are also some kinds of assets that can be problematic, to say it nicely. Everyone loves extensive grassy fields, woods, and lots of fences. These splines, as have been mentioned, don't draw well on the computer. It's due to the way they're made. Over the years, I have avoided grass splines for this reason. There are some newer ones which render much better, but you need to test them. Put them on a route, and if you get choppy performance in that area, remove them. Lots of trees, especially now the older billboard/flip board trees can be problematic. This is due to how they are handled by the computer. If you have a slower CPU and are tight on RAM, you'll have problems with these due to them being processed mostly by the CPU before being displayed. They also now have rendering issues which, render them, pun not intended, rather useless. Having too many Speed Trees can be an issue as well, especially if you have a slow video card. These are rendered in the video memory by the GPU (not CPU) mostly, and if your video is slow, they can make things slower.
One of the things to keep in mind,is if you have slow performance in an area without any trains, then you'll surely have worse performance with the trains. Train cars generally have a lot more polygons for the very reason to make them look good. Let's face it you don't want triangular tank cars! These extra polygons take extra work, and will only add to the slowness of an already overloaded scene.
In closing it all boils down to compromising, and finding what looks good, but still achieves a decent performance.
You do have my sympathy. I've had those before myself a few times and have passed them successfully. My dad and grandfather weren't so lucky and required surgery.
John
