Hi
If you're thinking in terms of your Immingham Docks project then I'm not convinced you really need to worry too much at all.
I'm not disagreeing with Barnez in any way, but the issues happen when tiles are loaded up and are more exaggerated the faster you're travelling. If you're bowling along at 100mph then there's a lot of processing power etc. happening to display all the assets as they pass by and it then seems to fall over itself trying to load up the new tiles. If you're on a 25mph route then you've got far more to play with.
The tile containing Weekday Cross and the London Road stations on my Netherfield route has over 2250 assets on it and it's never caused any problems whatsover that I can think of, because the speeds are so low.
Where you might want to be a bit more careful is where you've got loads of track and pointwork, and then you're in Woodhead territory.

Even having said that, in most of the WH yards the OHLE is made up of individual components which bumped up the asset count massively!
If that is of any help...
Cheers
Bob
(Your route's looking brilliant, by the way)