Encoding times. You can't squeeze more in less space without cost, cost here are cpu cycles. Encoding times of higher quality stuff which i did was about 5-6 times longer than comparable quality of x264. But it's size was around 40-50% smaller than x264 one. There is a cost not only in encoding, there is a cost in decoding too, it takes a lot more cpu cycles to decode x265. Either way HEVC is VERY GOOD codec.