'bin/logstash -f logstash.conf --configtest' takes ages to execute

I was testing Logstash 5.0.0-alpha2 in my CentOS Linux release 7.2.1511 (Core) VM with 8 GB RAM and 4 VCPUs.
I have installed java version "1.8.0_92"
Java(TM) SE Runtime Environment (build 1.8.0_92-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.92-b14, mixed mode)

When I execute (with or without the --debug):
/opt/logstash/bin/logstash -f /etc/logstash/conf.d/logstash.conf --configtest --debug
it takes 4 minutes and 36 seconds to execute. It seems to block mostly at this line:

config LogStash::Filters::Grok/@overwrite = [] {:level=>:debug, :file=>"logstash/config/mixin.rb", :line=>"154", :method=>"config_init"}

Is this a bug or is there something wrong in my configuration?

My logstash.conf contains:

input {
beats {
port => 5044
filter {
grok {
match => { "message" => "[%{HTTPDATE:timestamp}] [%{WORD:db_name}] [%{WORD:log_level}] [PID:%{WORD:PID}] [%{GREEDYDATA:source_location}] %{GREEDYDATA:log_message}"}
output {
elasticsearch {
hosts => "xxxx.cern.ch:9200"
manage_template => false
index => "%{[@metadata][beat]}-%{+YYYY.MM.dd}"
document_type => "%{[@metadata][type]}"