This sounds like a bug.
However, as I cross posted in other threads, the whole browser thing is a little limited in design.
As a developer, I can tell you that if it takes that long in any recently-ish hardware (less than 5 years old), it's all about the software, not because the machine is "slow" or because you "need an SSD".
If Reason needs to read a lot of metadata from each sample/preset file, then the Props should cache those (they almost never change anyway), and add indexes for quicker search / retrieval. It's not rocket science.
Probably they just used the lazy way out, and used the basic OS file operations, scanning each file individually, etc, but that, while it could have been "good enough" back in the day, it's not really good today where we have with tons of presets and samples.
I'm sure they know it, and will overhaul the browser at some point (7.5? 8?).
Another lazy cop-out pain point is the bitmap, non resizable, UI. Good enough when we had 72 dpi monitors back in 2001, not good anymore in the hi-dpi era -- it makes everything tiny. With a Retina it should even be totally unusable.