Hi Paul,
The 3D Warehouse contains many finely detailed works that perform well as standalone models.
Unfortunately all that minute detail makes them far too heavy to be used in a larger model.
There is no way to magically reduce the component’s weight on your model.
You need to ‘remodel’ the component, find a less complex version or simply model your own crane.
Creating (or downloading) overly complex geometry bloats the file and slows performance.
This topic may be of interest to you in terms of modeling technique and performance:
-Geo