Slow KahaDB access

Hi,
How frequently are these slow kahadb messages are logged?

I checked source code and found that these are logged during cleanup,
journal append and journal read.
I know cleanup happens only after cleanup interval but I know nothing about
how frequently journal append and journal read happens.
Does journal append and read happens for every new persistent message that
is received by the broker? or it happens after some time interval, if yes
can it be configured?

Thanks,
Abhi