We are using latest board, hardware is 8 core Xeon with 8G memory and M2 SSD ... top online around 150 users. But it takes ages, that "online" page produce results.

I made various optimisations found in this sub forum and our forum generally works well, except this. I found this in /var/log/mysql/mysql-slow.log 

SELECT count(*) FROM `armcore_sessions` AS `core_sessions` WHERE ( core_sessions.id IN(SELECT MAX(id) FROM `armcore_sessions` AS `s` WHERE s.running_time>1488722103 AND s.login_type!=3 GROUP BY `member_id`) OR ( core_sessions.member_id IS NULL AND core_sessions.login_type != 3 AND core_sessions.running_time > 1488722103) )

Any hint how to fix it?

It looks like fixed now. Recreating indexes wasn't any good ... recreating table with blank data was.

After a while is slow again.