Question: 11.3.2 Security Update on 2011 MacBook Pro kills performance
I have down graded to Sierra 3 times and things run fast again, as soon as 11.3.2 is installed the CPU is throttled down to very low clock rate on cold boot, to get it to come back up to speed you have to go into full sleep and wake it up. Its noticeably a little slower but running faster again.
i.e. Cold boot from downgraded (to Sierra) pre update boots FAST!!! after the update it is like a 300 baud modem in 1972.
There is something wrong in the patch which doesn't wake older CPU's to uptick. This continues on in High Sierra, so to make your old 2011-2013 Macs run fast downgrade, although I wouldn't use the machine online.