6.x works, but is extremely slow (in random cases) on larger wrml files
I suppose this could be due to extremely large number of objects, each of them consuming memory. As Win7 kernel and autorun software consumes more memory than Vista, the physical memory available to applications is somewhat smaller. When you run out of physical memory, intensive swapping of memory pages begins, which dramatically slows rendering.
For example, a simple ImageTexture node with a rather small image could require about 128 kB when it is instanced. In the scene with lots of identical textures (e.g. 10 000) all of them will consume more than 1 Gb of memory - if you have less physical memory available, it will cause swapping and a 60-100 times slowdown of rendering (that is according to my measurements - a same scene with high memory requirements renders 35 seconds on a machine with 2 Gb memory and 35 minutes on machine with 512 Mb).
If you instance the node only once, and then use the DEF/USE statements for each its appearance in the scene, you could save lots of memory - instead of creating 10 000 nodes, only one is created, and a lot less memory is used.
Using PROTOs, by the way, does not save memory.
Windows 7