troubleshooting Finder (and disk access?) speeds — Dropbox extension problems? I am on Yosemite 10.10.3, though in the public beta program (which now is in sync with releases). I am also in the Dropbox beta, now on 3.5.58, for what it's worth.

File access is curiously slow now. It could be a hardware issue, like the old spinning HDD dying in my MBP. That said, things often stall when Save or Open dialog boxes open up, and more often than not the Dropbox extension crashes afterwards.

Displaying the contents of folders is also slow, in the dialog boxes or Finder windows themselves, like in this question.

How should one start diagnosing such an issue?

I understand I could forget the Dropbox beta. I could also talk to them. But this has been crashing ever since Yosemite went to public beta testing, and no update to the OS nor Dropbox ever fixed this. The Dropbox beta is not that far ahead from the release anyway.

Is there anything to check here other than a reinstall or even a migration to a new mac?

I regularly run Onyx, so everything what it would do (incl. permission repairs) has been done recently. That said, Onyx's hardware check regularly complains about the disk, which needs to repaired after booting from the recovery disk. I don't know why this is recurring. Esp. as the problem is usually the disk size being off by a little.

[I'd paste screenshots of Disk Utility from booting into internet recovery, but they do not upload. Here are two links to them on Dropbox.]

I checked system.log, and saw no I/O errors, though most of my log is filled with these lines every second:

Apr 23 15:07:23 dhcp-7-125 com.apple.xpc.launchd[1] (com.apple.photostream-agent[92911]): LaunchServices returned a bundle URL that does not match with the LoginItem's known association.
Apr 23 15:07:23 dhcp-7-125 com.apple.xpc.launchd[1] (com.apple.photostream-agent): Service only ran for 0 seconds. Pushing respawn out by 10 seconds.
Apr 23 15:07:23 dhcp-7-125.nber.org com.apple.usbmuxd[49806]: HandleUSBMuxDictionary client 0x100608890-duet/NULL using library 1.2.5, running usbmuxd-344.6
Apr 23 15:07:23 dhcp-7-125.nber.org com.apple.usbmuxd[49806]: MuxTCPInputSCE received RST for 0x11-b22be791bfd836141f0489b3045fe82d713378b2@0xfd120000:2345-&gt;0x100608890-duet/NULL:33702: handleConnectResult: failure - 61


If this can be related to the Dropbox issues, here is another extension crash report from this morning. (Too large to show here, link is to a txt on Dropbox.) I've been sending these to Apple since last summer or so, neither OS X updates or Dropbox betas and releases seem to have fixed the issue.
Kill the service from terminal:

$ launchctl remove com.apple.photostream-agent


Then go into Apple / System Preferences... and check your iCloud settings. Often this is caused if iCloud photos or disk is on, but the Photos app has sharing off.