That is fantastic news. Every single time my Nexus 6P gets an update it feels like it takes at least 15 minutes to "optimize apps". Every. Single. Update.
The effect will be even more amazing for non-nexus/carrier devices with bloatware - my work phone (VZW M8) just got Marshmallow and it took couple hours to complete - most of the time was spent on twice optimizing 303 apps! (Only 30-35 out of those are things I installed.)