I recall reading something about this in the PostgreSQL mailing list, message written in 2016 but may still be relevant

https://www.postgresql.org/message-id/20151222124018.bee10b6...

“There's no substance to these claims. Chasing the links around we finally find this article: http://www.sqlskills.com/blogs/kimberly/guids-as-primary-key... which makes the reasonable argument that random primary keys can cause performance robbing fragmentation on clustered indexes.

But Postgres doesn't _have_ clustered indexes, so that article doesn't apply at all. The other authors appear to have missed this important point.

One could make the argument that the index itself becomming fragmented could cause some performance degredation, but I've yet to see any convincing evidence that index fragmentation produces any measurable performance issues (my own experiments have been inconclusive).”