Horrible trigger performance after upgrade 8.0.12 -> 8.2.3

After upgrading to 8.2.3 INSERTs and UPDATEs on one of my tables became
incredibly slow.  I traced the problem to one of my triggers that calls
one of my defined functions (that is IMMUTABLE).  If I inline the
function instead of calling it the runtime for my test update drops from
  10261.234 ms to 564.094 ms.  The time running the trigger itself
dropped from 9749.910 to 99.504.

BTW does make any sense to bother marking trigger functions as STABLE or
IMMUTABLE?