Why adding BEFORE TRIGGER affect client CPU?

We are running 9.6.8-10 on our five postgres clusters.
We have ~25 database per cluster.
We have two servers that update a table in each database.

During the update process we were running a pgpsql function that calculated
the difference in the new value with the previous. This function would do
a SELECT to get the previous value from the table and do the calculation
before the UPDATE. We also have an AFTER TRIGGER to save the row into a
logging table.

We recently changed the process to do a BEFORE TRIGGER to do the
calculation between OLD and NEW instead of the separate function with the
SELECT.

After doing this, CPU on our two client servers went crazy. CPU on the
database servers look unchanged. Rolling back this change fixed our client
CPU issue.

Anyone know of a reason why this change would affect the client? I would
think that this would only affect the server side. I could not see any
messages printed out anywhere in logs showing any type of error.

Thanks,
George Woodring
iGLASS Networks
www.iglass.net