Do you really have three monitors connected?
Anyway, if you do not have a very high resolution display (2560x1440 is not high resolution in this definition), you probably shouldn't enable the GPU acceleration. It really only helps if you have low megapixel images on a 4K or bigger display and do not do any brushing on your images. If you do any brushing definitely leave it off as it will slow down Lightroom terribly.
Turn off the GPU acceleration. Go to Preferences/ click on the Performance tab, then uncheck the box.
Yes, there are three monitors connected. The primary LR window is on a single screen however, so it's << 4K. I didn't know it only affects performance when working on screens with such high resolution. Well, I turned it off anyways. I had it turned off since the day LR6 was released, as I was already running into these freezes back then. But I thought it was due to the AMD driver and I wanted to give it another go now, after enough time has passed to expect working drivers from AMD and bug fixes from Adobe... Well, obviously not. Will keep it switched off. Thanks for the replies, guys! *thumbs up*