[russss]

Most reasonable databases do this very quickly. Postgres specifically can hold a cache of a number of sequence values on a per-process basis so each insert doesn't have to acquire a global lock on the sequence.

The disadvantage of this is that sequences in Postgres will have gaps in (as sequence ids aren't reused if transactions are rolled back).

That said, one good reason why centralized global sequences are not ideal, especially in very large systems where consistency is not paramount, is that they tie you to a single point of failure. In those cases it's better to implement a distributed sequence generator (of which GUIDs are probably the simplest type).