I got the problem too. When I activate "Camera Imager" in the Octane camera tag, the viewport becomes extremely slow. With deactivated "Camera Imager" everything runs smoothly.
Windows 10 64Bit
48 GByte Ram
2x GTX 980 (driver version 391.01)
Cinema 4D R19.053
Octane for C4D 3.08-RC1