Whoever told you that was mistaken.
In After Effects, you can choose (with the Continuously Rasterize switch) whether to treat the Illustrator object as a raster image or as vectors to rasterize for each frame. It's up to you, and you don't need to do anything special in Illustrator.
If you want to animate the individual items as layers in After Effects, you do need to make sure that each item is a top-level layer in Illustrator, but that's not what you were asking about.
See this page:
Thanks but that's not what I'm actually asking.
I'm asking the following -
I have a top layer in illustrator which has many sub layers and groups with lots of fx and symbols etc.
I am only importing as 1 layer to after effects.
But does after effects still se all the underlying information and have to render it all etc or does it just see it as a simple single vector shape/layer
Also in regards to your comment about the choice to import as raster. How do I do that? Thanks
If you're seeing everything in your drawing, seeing all of the layers, then so is AE. It renders them all. It uses all of the vector shapes as vectors except for those layers that have raster effects applied. A raster effect is any effect applied in illustrator that uses the document raster settings to make it's calculations. These layers are not continuously rasterized even when the CR switch is on.
Unless I specifically do not want to do any layer manipulation of the individual layers I always use the release the layers tool to give me individual layers for each shape. Some of these shapes may be re-grouped. Some may not. Then I import the .ai file as a composition retaining layer sizes. This allows me to do all of the manipulation that I need in AE.
Also, if I am given an .ai file that has a bunch of raster effects applied I try and remove those in a duplicate copy and then use the original as a reference to rebuild the file in AE. I actually have to do this fairly often with logos designed by a print person.
I hope this helps. Generally .ai files render very quickly unless the number of line or triangle segments go way way up. Unless you are experiencing render problems I would not worry about the groups and effects.