Base 2015 27" Retina iMac very laggy

Discussion in 'iMac' started by weezin, Dec 15, 2018.

  1. weezin macrumors regular

    Jul 20, 2012
    Want help confirming my suspicions!

    My work computer is a base 2015 Retina 27" iMac running Mojave (previously running Sierra). 8gb of ram, mechanical HDD. I forget the processor.

    I find the machine to be quite laggy and slow at times. The slow performance is not consistent, it comes and goes. Opening spotlight can take ~3 seconds and inputting text can be laggy. Right clicking a file can take a couple seconds for the menu to come up. Opening Photoshop CS6 can take ~45 seconds. Chrome loads slowly. Pages are slow to render.

    I primarily work in Chrome (email, github, several tabs, etc), Slack, Figma and very occasionally Photoshop (I usually have this closed).

    My guess is the slowness is coming from the HDD. The processors should be up to the task (easily I would think), and the ram doesn't seem to be a problem (I have checked the memory pressure and that seems consistently to be fine). I can't imaging the video card presenting an issue as well.

    Does that make sense? I'm a bit surprised at how laggy it can be even considering the mechanical HDD but would guess that adding an SSD boot drive would speed it up significantly. For reference, my 2011 27" iMac (with SSD) runs circles around this machine for regular tasks.
  2. Lunder89 macrumors 6502


    Oct 16, 2014
    I have mid-tier 2015 iMac 5K, and I ordered mine with the SSD drive, and it runs very smooth, and very fast. Your suspicions about the harddrive are correct. A Mac running Yosemite and above with a classic spinning drive, are slow as heck at anything.

    The mechanical harddrives in the iMacs are not very good. They are 7200 RPM, but they only have 32MB cache, which is what I used when I built gaming PC's in 2007 and 2008. A higher cache often yields a better multitasking performance.
    In a sequential read/write they can reach 100 MB/s maybe 120 MB/s. But for reading and writing allover the place its closer to 20-30 MB/s. The pure SSD drive for that model can reach something like 1,5 GB/s for sequential read/write. And get very close to that for multitasking.

    And macOS is always doing something. Either with a SWAP file, a cleaning task, or just running all the fancy features for hand-off and whatnot. Basically something is always accessing the drive. Meaning classic HDDs make for a terrible experience. Apples execs should receive a slap every morning for still selling their Macs that way :).
  3. weezin thread starter macrumors regular

    Jul 20, 2012
    Great, that helps, thanks! It is likely as I suspected - now to figure out what to do with the machine. Taking it to a third-party repair place would probably be the deal (as I doubt my company wants me working on it).

    Thanks again!
  4. mikehalloran macrumors 65816

    Oct 14, 2018
    The Sillie Con Valley
    Your iMac should have a 1TB HDD now. You have 3 choices:

    a) Replace the HDD with an SSD on the SATA III buss. It will run circles around that old HDD. The Samsung 860 EVO is available from 256GB to 4TB. This is an inexpensive upgrade that will take a tech about an hour or less.

    b) Put a Samsung 970 EVO up to 2TB on the PCIe buss. This will run about 6x faster than an SSD on the SATA III buss. Takes about an hour and costs about $200 more than (a). This is what you want your company to approve—far le$$ than a new 1T SSD iMac and nearly as fast.

    c) Do both of the above. No reason unless you really need up to 6TB total storage.

    I did not list replacing the original HDD with the same. With the low prices on both types of SSDs, that's just silly.
  5. dobrink macrumors member


    Feb 4, 2013
    Helsinki, Finland
    Can you please link me to guides on how to install these? However, at this point I am mostly interested in what equipment I need to gather before I start the work. So far I got:

    For a) Samsung 860 Evo, prying tools, screwdrivers, 2.5" SSD to 3.5" bracket, adhesive strips for closing the iMac
    I think I only need some thermal sensor now, but is it 100% necessary? What about if I place the SSD just next to the Fusion with double sided tape or something, is it possible to have two drives, do I need some special connector or cable for that?

    For b) a Samsung 970 Evo M.2 PCI-E
    Do I need any adapter in order to place it where the original blade SSD is ?

    P.S. I own a late 2013 27" iMac and a late 2015 27" iMac, with their original Fusion drives.
  6. Fishrrman macrumors P6


    Feb 20, 2009
    OP wrote:
    "My guess is the slowness is coming from the HDD."

    That's the problem.

    "I'm a bit surprised at how laggy it can be even considering the mechanical HDD but would guess that adding an SSD boot drive would speed it up significantly."

    THAT'S the solution.

    Mike Halloran offers one way to do it in reply 4 above.
    You'll have to open the thing up yourself, or have someone do it for you.

    If you don't want to open it up, you can buy a USB3 EXTERNAL SSD, plug it in and set it up to become the boot drive.
    It won't be quite as fast as an internally-installed SSD, but ought to come fairly close (say, 80-85%).
    You don't need a large external SSD -- 500gb or even 250gb can do the job.
    Set it up with OS, apps, and basic accounts.
    Leave "large libraries" of stuff on the internal drive (movies, music, pics).
    You can velcro the SSD to the back of the iMac stand, up and out-of-the-way.

    A Samsung t5 or Sandisk Extreme are good candidates, cheap and fast.

    Either an "internal" or "external" solution will work.
    It's up to you.
  7. weezin thread starter macrumors regular

    Jul 20, 2012
    Thanks! Seems like a) is hte way to go for us. We'll see what happens!

    Thanks, much appreciated!
  8. mj_ macrumors 6502a


    May 18, 2017
    Austin, TX
    There's also a third option c) that does not require opening the iMac and will be 95% as fast as option a)

    Get an external USB 3 SSD, like the Samsung T5, and run your iMac off of that drive.
  9. Moonjumper macrumors 68000


    Jun 20, 2009
    Lincoln, UK
    I have a separate install of macOS on an external SSD. I have no problems with performance when using it. Probably slightly faster than my internal Fusion set up (128GB SSD + 3TB HDD).

    You can leave everything as it is on the internal drive and access it if you have an external boot drive (I’ve done it as I need to update to Mohave soon for app uploads, but the external will stay on High Sierra for legacy software).

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8 December 15, 2018