There is huge delay starting up logstash 5.0.0-alpha5

We're using Elastic Stack 5.0.0-alpha5.
We have very big trouble starting up Logstash.

Logstash won't start up after executing (as a service) over 1+ hours.
Process is alive, but there's no messages or logs even with debug configuration.

Are there anyone know the reasons and workarounds?

Running environment:

Ubuntu 16.04 LTS with java version 1.8.0_101
ElasticStack 5.0.0-alpha5

How are you starting it?

Starting with "systemctl start logstash.service"

So how exactly do you know it takes an hour to start?

I'm looking into logstash log file with wrong configuration file.

What does that mean?

That means I used wrong configuration file intentionally to check start up delay time.
Logstash logs error about wrong configuration file.

Also, I tested with logstash 2.4.0 on the same machine, and it worked fine.