What's the cause of low hashrate on Radeon RX 580 4GB?

Question

I've just started mining Ethereum Classic with Sapphire Nitro+ Radeon RX 580 4GB and lolMiner. While some people have reported that the hashrate should be about 30 MH/s, on my system it doesn't go beyond 14 MH/s (as reported by lolMiner). So the question is: why?

The motherboard is very old. It's an ASUS Commando with 4GB of DDR2 RAM. I hoped that the transfer of data between the RAM and the GPU is minimum and, thus, the quite low speed of the memory bus doesn't affect the hashrate significantly. But now my doubts are stronger.

Other details:

1. OS is Windows 7 64bit (updates up to 2017 are installed).
2. The most recent AMD GPU driver is installed.
3. The system has only one graphics card, which is also used for display.
4. GPU-Z reports that "CPU clock = 1411 MHz", "Memory clock = 1750 MHz", "GPU Load = %100", "Dedicated Memory Used = 3936 MB", "Dynamic Memory Used = 217 MB", and "Memory Controller Load = %68".

Answer 1

I reached a hashrate of 28 MH/s and don't want to increase it further. My problem was solved when I installed Windows 10. On Windows 7 the compute mode option in Radeon Software was missing. When I activated compute mode, the hashrate increased to 22 MH/s. Then by tuning the graphics card the hashrate increased another 6 MH/s.