I am using Assimp to import some 3d models.
Assimp is great, but it stores everything in a non-interleaved vertex format.
According to the Apple OpenGL ES Programming Guide, interleaved vertex data is preferred on ios: https://developer.apple.com/library/ios/#documentation/3DDrawing/Conceptual/OpenGLES_ProgrammingGuide/TechniquesforWorkingwithVertexData/TechniquesforWorkingwithVertexData.html#//apple_ref/doc/uid/TP40008793-CH107-SW8
I am using VertexArrays to consolidate all the buffer related state changes – is it still worth the effort to interleave all the vertex data?
Because interleaved vertex data increases the locality of vertex data, it allows the GPU to cache much more efficiently and generally to be a lot lighter on memory bandwidth at that stage in the pipeline.
How much difference it makes obviously depends on a bunch of other factors — whether memory access is a bottleneck (though it usually is, since texturing is read intensive), how spaced out your vertex data is if not interleaved and the specifics of how that particular GPU does fetching and caching.
Uploading multiple vertex buffers and bundling them into a vertex array would in theory allow the driver to perform this optimisation behind your back (either so as to duplicate memory or once it becomes reasonably confident that the buffers is the array aren’t generally in use elsewhere) but I’m not confident that it will. But the other way around to look at it is that you should be able to make the optimisation yourself at the very end of your data pipeline, so you needn’t plan in advance for it or change your toolset. It’s an optimisation so if it’s significant work to implement then the general rule against premature optimisation applies — wait until you have hard performance data.