I'm facing the same problems.
Pages loading time is higher, using scroll makes me shoot my head and almost all the times when page is loading the fun is at full speed. I have the version 38.0.2220.12, and with the earlier version everything went fine! I'm considering to say goodbye to opera beta
For some reason the Beta seems to rely more on the CPU for very simple operations like page loading. The freezing, at least in my case, is definitely cause by CPU spikes (see images above). The Dev Opera (v. 39) helped a bit, but there are still spikes.
Worse, graphically intense pages (many small videos/pics) are still very bad and at this time I simply have to switch to Edge for smooth scrolling.
I hope this gets remedied, I was just getting to like Opera again after a years long hiatus when they ditched the old engine.