You do mention that you have specified the opencl switch at start.
Did you specify -G in the ethminer parameters? This is required for ethminer to mine using your GPU. Here's the relevant ethminer --help :
Mining configuration:
-C,--cpu When mining, use the CPU.
-G,--opencl When mining use the GPU via OpenCL.
--opencl-platform <n> When mining using -G/--opencl use OpenCL platform n (default: 0).
--opencl-device <n> When mining using -G/--opencl use OpenCL device n (default: 0).
-t, --mining-threads <n> Limit number of CPU/GPU miners to n (default: use everything available on selected platform)
--allow-opencl-cpu Allows CPU to be considered as an OpenCL device if the OpenCL platform supports it.
You will get about 15 lines of messages that OpenCL is initialising if the switch works correctly - the same messages should be displayed when you are running the benchmark. Here's what it looks like:
[OPENCL]:Found suitable OpenCL device [Hawaii] with 8513388544 bytes of GPU memory
[OPENCL]:Using platform: AMD Accelerated Parallel Processing
[OPENCL]:Using device: Hawaii(OpenCL 2.0 AMD-APP (1800.11))
[OPENCL]:Printing program log
[OPENCL]:
[OPENCL]:Creating one big buffer for the DAG
[OPENCL]:Loading single big chunk kernels
[OPENCL]:Mapping one big chunk.
[OPENCL]:Creating buffer for header.
[OPENCL]:Creating mining buffer 0
[OPENCL]:Creating mining buffer 1
[OPENCL]:Printing program log
[OPENCL]:
[OPENCL]:Creating one big buffer for the DAG
[OPENCL]:Loading single big chunk kernels
With ATI GPU, your CPU is used heavily during the dag creation phase but after that, there's almost no impact on CPU except the whole computer heat generated. Also disk access and network are lightly used during mining. I think you just should try and see it by yourself but if your computer is well cooled, you should be ok.
Best Answer
it does seem the above mentioned lists are no longer available. so try this one, its up to date and recent. GPU hash comparisons