Hey @magnusbaeck,
Earlier we kept that conf file which I have sent you in /etc/logstash/conf.d directory but that time also CPU usage was high so for testing purpose we removed that .conf file and kept CPU idle.
I have done a setup on one more redhat server and kept below .conf file in /etc/logstash/conf.d directory but there is also CPU usage is high.
I am using below type of .conf file.
apache-access-logstash.conf
https://pastebin.com/raw/xqBuSdD9
apache-error-logstash.conf