4 core & 4 threads works fine, 4 core only is definitely an issue. Overclock helps also.
I started with an i5 3570k, 4 core at 4.4 ghz at 100%. Changed to i7 3770k at 4.4 ghz and the extra 4 threads made a significant difference.
I’m still amazed this software runs really well on an 8 year old cpu