With the addition of ContextualExecutors to Servlet3AsynchronousResponses, in Non-EE environments the resteasy.async.timeout.scheduler.min.pool.size config is always looked up while constructing the Response. And the way its done is by trying to load an implementation of the ConfigurationFactory and this in-turn causes the Threads to get blocked while looking up the implementation (shown in the stack-trace attached).

Loading the config ahead of time in a static way seems to resolve this.

JDK Used: Java 1.8 u 232.