Performance issue after upgrading from 9.4 to 9.6

We are having some performance issues after we upgraded to newest
version of PostgreSQL, before it everything was fast and smooth.

Upgrade was done by pg_upgrade from 9.4 directly do 9.6.1. Now we
upgraded to 9.6.2 with no improvement.

Some information about our setup: Freebsd, Solaris (SmartOS), simple
master-slave using streaming replication.

Problem:
Very high system CPU when master is streaming replication data, CPU
goes up to 77%. Only one process is generating this load, it's a
postgresql startup process. When I attached a truss to this process I
saw a lot o read calls with almost the same number of errors (EAGAIN).

root(at)d8:~ # truss -c -p 38091
^Csyscall seconds calls errors
semop 0.001611782 198 0
write 0.000074404 2 0
read 2.281535100 17266 12375
openat 0.000683532 48 0
lseek 0.177612479 20443 0
close 0.000395549 48 0
------------- ------- -------
2.461912846 38005 12375

read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(444,0x0,SEEK_END) = 32571392 (0x1f10000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(444,0x0,SEEK_END) = 32571392 (0x1f10000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(7,0x0,SEEK_END) = 164487168 (0x9cde000)
lseek(778,0x0,SEEK_END) = 57344 (0xe000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(894,0x0,SEEK_END) = 139296768 (0x84d8000)
lseek(894,0x4b7e000,SEEK_SET) = 79159296 (0x4b7e000)
read(894," ~\0\08\a\M--m\0\0\^A\0\M^T\0000"...,8192) = 8192 (0x2000)
lseek(3,0xfa6000,SEEK_SET) = 16408576 (0xfa6000)
read(3,"\M^S\M-P\^E\0\^A\0\0\0\0`\M-z"...,8192) = 8192 (0x2000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(894,0x0,SEEK_END) = 139296768 (0x84d8000)
lseek(894,0x0,SEEK_END) = 139296768 (0x84d8000)
lseek(894,0x449c000,SEEK_SET) = 71942144 (0x449c000)
read(894,"\^_~\0\0\M-H\M-H\M-B\M-b\0\0\^E"...,8192) = 8192 (0x2000)
lseek(818,0x0,SEEK_END) = 57344 (0xe000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'
lseek(442,0x0,SEEK_END) = 10174464 (0x9b4000)
lseek(442,0x4c4000,SEEK_SET) = 4997120 (0x4c4000)
read(442,"\^_~\0\0\M-P\M-+\M-1\M-b\0\0\0\0"...,8192) = 8192 (0x2000)
read(6,0x7fffffffa0c7,1) ERR#35 'Resource temporarily unavailable'

Descriptor 6 is a pipe

Read call try to read one byte over and over, I looked up to source
code and I think this file is responsible for this behavior
src/backend/storage/ipc/latch.c. There was no such file in 9.4.

--
Piotr Gasidło