If by ‘library load time’ you mean the time it takes from reboot to the Roon jellyfish disappearing I think your times look rather good for such a huge library.
With 150k tracks, external USB storage on a 7i7NUC, it is about 2 minutes for me. Library scan time finishing very shortly after. Once scanning is finished, everything is reasonably snappy but I think we have all become accustomed to the small ‘cloud search’ delays introduced with Roon 2.0. But in-use performance is another topic entirely and as noted in previous posts, some large libraries have performance issues by design (under investigation).
See the following which may or may not be related to your problem: