I'm currently using Xcode 10.1 and it's driving me crazy, it slows my computer down to a point that it is unusable. Before I upgraded to Xcode 10 I was running the same project on Xcode 9 and High Sierra without any problems and with only half of the memory it currently has (8GB of Ram). I now have upgraded the memory to 16GB but it didn't make a difference.

FYI - I don't think the slow down has to do with Mojave 10.14.2 since I was experiencing the same issue with High Sierra and Xcode 10, which is the reason I upgraded to Mojave thinking that Xcode 10 would run smoother in the newest operating system.

Here are the specs of my computer:

Computer has a fusion drive.

screenshot of computer specs

Here is what I see in Activity Monitor for CPU and Memory usage:

screenshot of activity monitor CPU tab screenshot of activity monitor memory tab

Disk Activity

Screenshot of activity monitor disk tab

Any idea what could be wrong?

EDIT: The Simulator may be the possible cause.

In the comments below I stated that the issue was resolved by uninstalling and reinstalling Xcode but I later found out that it wasn't true. The issue seems to be caused by the Simulator, what I noticed is that if I reboot my computer and don't use the Simulator, Xcode runs fine. But as soon as I use the Simulator, Xcode starts running slow even after closing the Simulator. I need to reboot and use Xcode without using the Simulator to temporarily fix the issue. I have been using hardware-devices for testing and Xcode seem to be running fine. Any thoughts?


  • 4
    Well, it doesn’t look as if Xcode is to blame, at least not based on the screenshots above. Can you provide the same screenshots from a moment where the system is close to unusable (take a picture with a smartphone if the system is too slow for screenshots)?
    – nohillside
    Dec 30, 2018 at 18:42
  • These screenshots were taken when the system was running slow. I’m just blaming XCode because if I close it everything starts running fine.
    – fs_tigre
    Dec 30, 2018 at 18:45
  • 1
    The system is 91% idle, there is no memory pressure (and hardly any swapping). If it's slow it's coming from something else. I assume you don't have an SSD in a 2012 iMac, what kind of disk activity is going on in such moments?
    – nohillside
    Dec 30, 2018 at 19:00
  • I just added a screenshot of the Disc Activity. The computer has a Fusion Drive.
    – fs_tigre
    Dec 30, 2018 at 19:12
  • 1
    Thanks, looking at it right now, and still wondering. None of the screenshots reflects the slowness you are describing. Speaking of which: what does "close to unusable" mean for you specifically?
    – nohillside
    Dec 30, 2018 at 19:15

1 Answer 1


I do have the same issue - unlike many other posts on the Web, I only have one view controller with a couple of slides and views, so simplifying is not a solution for me -

I suspect the simulator seems to slow things down - there is a SimStreamProcessorService which seems to take a lot of CPU time (look at Activity Monitor under %CPU) -

I restarted XCode and didn't use the simulator and it seems to be running fine. I have a MacBookPro late 2011 with 16GB RAM running Sierra -

  • As I stated in my Edited question: For me restarting XCode doesn't fix the issue, I need to reboot my computer and use XCode without using the Simulator. What I have been doing as a workaround is using my iPhone for testing and XCode runs fine.
    – fs_tigre
    Mar 15, 2019 at 12:38

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