It seems to be an easy task, make available logging from haproxy to syslog. Configuration on haproxy side was pretty easy, log file start growing, but from time to time there were some pauses. Rsyslog was the first victim, I dig into limiting configuration area, but default limits doesn't apply in this particular situation. After some time I realized that this configuration also includes systemd-journal which catch-all syslog data and then send its to rsyslog. At systemd-journal level there is also ratelimit configuration and that was the point in my case.