Oh boy I just looked at my previous post when I had to reinstall and this issue actually started at 4.5 and has just gotten gradually worse, I am fairly certain I know what it is. It is probably the DDC. This is what I am thinking is happening anyway. It takes along time to import new stuff, much longer than it use to. I always thought it was referencing everything and that what was taking so long.
For example: a character Model use to take about 30 to 40 seconds. Now days it takes anywhere from 30 seconds to 5 minutes depending on the size and the complexity. However maybe it is not Caching the materials/textures. So when I load the asset the first time it has to do that which sometimes is staggering how a 512X512 texture can take so long to cache.
I suppose in 4.1 to 4.4 I got use to placing a freshly imported object and that object having a world grid material temporary placed on it while it cached. Now the editor freezes while the object gets sorted into the content browser and referenced.
There is also the option that it is just taking a long time to load the mats. Yesterday I placed a box with middle of the line resolution textures and it took about 5 minutes of the editor being frozen. The editor was using all of my CPU and about 5gigs of my memory. It kept becoming unresponsive because during that process it started building shaders right in the middle of the import.
The shaders are a big issue and always have been because they just start building adding processing power whenever. But they only seem to melt down the editor when I am building lighting and they go off or when I am importing something.