@alberto, it says it is a dual pc. (2 cores).
Most advised thing is to set the number of theads twice the number of cores (2xNcores= theads), you might want to try that and see how it affects your mining performance.
You can set it on the json file, set 4 and see how it goes, then 3... or 2. It's up to your processing power.
I tried that and now it helps. I just wonder why the json file comes with a "null" value there :/