Is making joins between different table spaces slower?
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).