1.5.8 shouldnt be faster than 1.5.9, cause nothing was changed that could impact speed. Placebo 
I thought there were my personal suspects, but checked again after reading this and yes, 1.5.8 is a little bit faster.1.5.9: "hashrate_total_30min": 7738
I've terminated it and run 1.5.8 with same configs: "hashrate_total_30min": 7746,
maybe new nicehash features add a tiny lag to a mining process?