Conventual wisdom is the overhead of write barriers is somewhere around 5-10%. There are tricks you can use to reduce it, though; you can coalesce the barriers

Conventual wisdom is the overhead of write barriers is somewhere around 5-10%. There are tricks you can use to reduce it, though; you can coalesce the barriers for objects together into one block of operations if you have a JIT, only do the write barrier if the GC is in a collecting state (and then the overhead is 0% most of the time!), skip the barrier if you know an object is short lived and doesn't escape a scope, etc.