I frequently need to transfer files between 10 GB and 1 TB as part of my work, typically between external drives and network servers. Whenever I transfer files, the computer slows down to being completely unusable. E.g. it can take five minutes to open a terminal window or browser, even if the application is already running.

Because of an "air gap" situation (isolated network) I have two Mac Minis, one with 16 GB RAM and one with 8 GB(not upgradable). The 16GB machine doesn't slow down much while transferring. The 8 GB mini is pretty slow in general, but works fine for mail, browsing. This one slows to a crawl, so in practice I can only start transfers when leaving the office. The transfer speed itself remains pretty OK -- typically 20MB/s limited by the remote server. I use FileZilla, scp or rsync, the computer slows down all the same.

I use different external drives, but it is also slow to transfer to/from the internal drive.

Is there any parameter I could adjust, and are others seeing this?

System / transfer parameters

  • Computer model: Mac Mini Late 2014 / 2.6 GHz / 8 GB 1600 DDR3
  • Operating system: macOS Mojave v. 10.14.3
  • Internal disk: 1 TB SATA 5400 RPM APPLE HDD HTS541010A9E662 formatted as Encrypted APFS
  • External disk 2 TB SATA formatted as Encrypted Case-Sensitive Journaled HFS+
  • Alternative external disk: 2 TB exFAT formatted (not encrypted)
  • Network protocol: SSH-based protocols (scp, sftp). Other protocols also seem to be affected, including HTTP download via the browser.

Before transfer:

activity monitor

Network I/O: activity monitor-network

After uploading for 15 minutes the computer is very slow. Memory status again:

activity monitor

But it seems to have improved a bit in comparison to how it was before. I'm able to edit this question and take screenshots etc. with only a little bit of patience. I believe it gets worse after a while though.

CPU usage during transfer depends on current speed, but it is not excessive: CPU usage about 20 %: primarily kernel task, fzsftp

  • Can you add some details about macOS versions in use, filesystem used on the external drives, network protocol used to acess network servers? And screenshots from Activity Monitor showing relevant details while the system appears to be sluggish? – nohillside Feb 12 '19 at 10:12
  • The first thing that I notice is that you've got a spinning internal drive (slow to begin with) that's encrypted and journaled external drives connected over USB which is also slow to begin with. The amount of decryption that is necessary here is probably taxing the CPU, which, by the way is what you didn't show in your screen shots. – Allan Feb 12 '19 at 16:34
  • The internal disk is indeed one of the slowest possible, but when not transferring files the slowness is quite controlled and predictable. I don't think CPU is the issue, but will update the question with screenshot from CPU, probably later today. – Marius Bjørnstad Feb 18 '19 at 10:32

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