When downloading a file at 2+ MB/sec (from yandex.net) Opera heavily eats CPU on my PC (1 core is utilized 50-80%), which makes the browser and other application hardly usable.
I usually download with Download Master, and it doesn't occupy CPU (at least this is hardly noticable) at same speeds.
Opera: 64.0.3417.92 (32-bit)
OS: Windows 7 64-bit SP1
PC: Lenovo B580 / Intel Core i3-2328M 2.20 GHz / 16 GB RAM
It seems to be a know issue, which presumably is (will be) fixed in 66 version.