For each placed Magnolia tree from the asset browser which has the same Magnolia CR material for all different Magnolia trees, C4D will still generate a new material slowing down everything.
Yeah, that's dumb but unfortunately, that's normal according to Maxon since materials are static, unlike objects which can be instantiated and linked. This kept me from building a large model library for a specific long-term project upon the AB and it's too bad since the deep level of integration is nice.
Aturtur wrote a script to merge materials sharing the same name, which is a bit unsafe since a name can be shared by as many materials as you want, even between materials from different render engines. I've modified it to run over the current material selection, which is a bit safer (also fixed a few bugs in the process). It's attached if you need it.