Is making joins between different table spaces slower? In MySQL and PostgreSQL, is making joins between different tablespaces inherently slower, compared to joining tables in the same tablespace?
This is only for Postgres:

If the tablespaces reside all on the same harddisk, then there is no performance difference at all. 

If the tablespaces are created on different harddisks then yes, this can make a difference. In that case the necessary I/O is spread over multiple disks, thus improving the overall throughput (assuming all harddisks have the same performance).