I noticed that when I use alot of gradients and blurring the program grinds to a crawl,
It would be a great feature if when I grouped a set of vectors, it put a rendered png picture of that group in its place until ungrouped.
to take the weight of the app, much like using low poly models in 3D to represent a high poly model? ok it may be a stupid idea, but its an idea.

Gradients shouldn't slow things down much, but blur will. You can lower the quality of the blur to speed things up again.

If you were thinking of doing this only to groups because they can't be edited until released, this is incorrect. You can still edit objects within a group, so you still need a live vector version to be able to edit it properly.

It has been discussed before to manually convert objects it to bitmaps and then hide the vector version as a way to increase performance. But I don't think it would work well if Inkscape automatically did this.