By now everyone would know that PostgreSQL 13 is the best version available for anyone.
One of the new features that worth mentioning is the way b-tree index is stored for duplicate values. If you have columns with low-cardinality and have duplicate values in them, Postgres will now store them only once. This means saving space and easier on the I/O for searching the values.
Because the performance gain during read is very real and so b-tree deduplication is very recommended. Deduplication is not applicable to some cases because of equal datums. But that’s practically nothing.
But, there is this part in the documentation:
Write-heavy workloads that don’t benefit from deduplication due to having few or no duplicate values in indexes will incur a small, fixed performance penalty ..
The problem is not about heavy-write workloads, but this is about MVCC.
Since deduplication is enabled by default, some users might be experiencing this.
The good thing is, Postgres has a method to disable deduplication for the specific index by setting deduplicate_items to off. You will not be benefiting from anything, but better than degraded performance.