"--opencl" only means using OpenCL for part of (currently lookahead) processing. It does not, and will never necessarily mean being faster. That's why when you try --fullhelp you won't find terms like faster, boost, accelerate, etc. in the official explanation of --opencl. The speed varies according to your hardwares, encoding settings, sources and many other things. OpenCL may even get it slower if the speed of data copy between GPU and CPU, or the speed of GPU is the bottleneck. So if someone tells you that using --opencl is faster without specifying in which case it is, one must be somehow irresponsible for one's words.