As a major version change, much of the code in 15.x is new. And as they upgrade the code, it normally means it needs more hardware to work well. It's a pain, but it's been that way with computer programs since we bought our first computer in the business in .... '87 or so. Long time ago.
I'm surprised a massive BRAW project runs even on a 14.x version with a 5400RPM external drive for the media. On my last desktop, replaced this winter, I had a 6-core rig with 32GB of RAM and six internal SSDs. Some media was on a standard spinner, the 7200RPM one, and even then ... an internal 7200RPM drive ... it could have troubles at times. Lagging playback and such.