Please update depencies SLF4J from 1.5.2 to the latest version which has lot of performance improvements.
SLF4J-API 1.5.2 works with SLF4J 1.7.5 implementation howether mixing APi and implementation different versions is not recommended by SLF4J project (http://www.slf4j.org/compatibility.html).
Changelog SLF4J : http://www.slf4j.org/news.html
Updated SLF4J and LogBack to latest versions.