I created from RAW files (D750) for the ones I've submitted. It was on a maxed out iMac 5k, though I was able to do a panorama on my Macbook Pro (2011 model with integrated graphics and slower HDD) without much of a delay (but to be fair, I only did a few and they were Sony a7s 12 megapixel shots though also from RAW)
Task manager sounds like you're using Windows. If so, did you have any background processes eating up memory? I know on any Windows machines I've ever used, I always had to go to msconfig to stop a lot of unnecessary start up programs. I think there are articles out there too for optimizing Lightroom which may be a good place to start or finding out if you can get anymore RAM installed.