There are periodic JFR events that calculate statistics for the string table and symbol table. These calculations are run in the in_vm thread state, and hence delay safepoints for the duration of the calculation. There is no incremental polling either, so the safepoint delay is proportional to the size of the tables. The events run by default every 10 seconds. I would class that as a performance bug. It is not expected and could have severe consequences.
Moreover, the string table can get very large and it isn't clear if doing this every 10 seconds is a good idea.