In most modern systems the rate at which the CPU can compress data is many, many times faster than the disk can write data. In general compression greatly increases write speeds because less disk IO is needed. The vast majority of benchmarks show that to be the case too.
TL:DR, if you notice a performance decrease after enabling compression, something is wrong with your NAS/SAN.
That said, if you are just storing large encrypted files on your ZFS then compression isn't needed.
In most modern systems the rate at which the CPU can compress data is many, many times faster than the disk can write data. In general compression greatly increases write speeds because less disk IO is needed. The vast majority of benchmarks show that to be the case too.
TL:DR, if you notice a performance decrease after enabling compression, something is wrong with your NAS/SAN.
That said, if you are just storing large encrypted files on your ZFS then compression isn't needed.