The logger factories in most SLF4J modules namely in jcl-over-slf4j, log4j-over-slf4j, slf4j-jcl, slf4j-jdk14, slf4j-log4j12, and slf4j-simple now use a ConcurrentHashMap instead of a regular HashMap to cache logger instances. This change significantly improves logger retrieval times at the cost of some memory overhead.

Acording to SLF4J developers:
Given the significance of these performance improvements, users are highly encouraged to migrate to SLF4J version 1.7.5 or later.