slow boot & disk management

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HI All,
I have two disks (both of them is Samsung, 750GB & 500GB.
sudo hdparm -i /dev/sda

/dev/sda:

Model=SAMSUNG HD753LJ, FwRev=1AA01113, SerialNo=S13UJ1NQ900065
Config={ Fixed }
RawCHS=16383/16/63, TrkSize=34902, SectSize=554, ECCbytes=4
BuffType=DualPortCache, BuffSize=unknown, MaxMultSect=16, MultSect=16
CurCHS=16383/16/63, CurSects=16514064, LBA=yes, LBAsects=1465147055
IORDY=on/off, tPIO={min:120,w/IORDY:120}, tDMA={min:120,rec:120}
PIO modes: pio0 pio1 pio2 pio3 pio4
DMA modes: mdma0 mdma1 mdma2
UDMA modes: udma0 udma1 udma2 udma3 udma4 udma5 *udma6
AdvancedPM=yes: unknown setting WriteCache=enabled
Drive conforms to: unknown: ATA/ATAPI-3,4,5,6,7
* signifies the current active mode
When booting systemd-analyze critical-chain also shows either the disk driver is not good or the disk configuration is not complete & competitive or my disks too old. Please suggest something to the disks can remain in use. Thanks.

graphical.target @20.729s
└─udisks2.service @12.354s +8.374s
  └─basic.target @11.774s
    └─sockets.target @11.774s
      └─uuidd.socket @11.774s
        └─sysinit.target @11.592s
          └─systemd-update-utmp.service @11.280s +311ms
            └─systemd-tmpfiles-setup.service @9.327s +1.901s
              └─local-fs.target @9.292s
                └─home.mount @8.897s +395ms
                  └─systemd-fsck@dev-disk-by\x2duuid-a55cf807\x2dbdcf\x2d416c\x2d8e9e\x2daba7ab49f22c.service @8.719s +165ms
                    └─dev-disk-by\x2duuid-a55cf807\x2dbdcf\x2d416c\x2d8e9e\x2daba7ab49f22c.device @8.717s

"I have two disks"

What is the purpose of these disks?
If you are able to spread the load equally between them, then this will accelerate your system.
One possible split is be having the root partition on one disk and the /home partition on the other one.
I am not sure whether this will be visible in boot speed, but it should increase disk access during operation of the system.

How often do you boot your system that you see the need for reducing the boot time, which is shown as 20 seconds in your systemd output?