Can sharding speed up insertion performance in postgresql
As has been clarified in the comments, accessing a sequence will not be a bottleneck in PostgreSQL.
If you are faster inserting into a sharded database depends on where the bottleneck is.
If it is disk I/O or CPU on the database, you will probably be faster spreading the workload across several databases (sharding), as long as they have independent storage and network bandwidth is not a bottleneck.
If the bottleneck is on the client side, you probably won't gain much.