KDE panel takes two minutes to appear after login

Question

I upgraded for Ubuntu 14.04 to 16.04, which comes with KDE 5. It now takes about two minutes for KDE to start from the login screen to a resposive KDE session. The apps show up from my saved session in about 30 seconds, the panel doesn't appear until 90 seconds, usually in a munched form (empty except for part of the clock, for instance), and the panel (and background) aren't responsive until the full two minutes.

By contrast, Unity is up within 5 seconds.

I've looked at this bug and tried most of the suggestions there as well as suggestions elsewhere (such as disabling baloo, disabling the spash screen, removing bluez and the network manager applet). None have any effect.

I also tried making a new user, in case something was wrong with the KDE setup on my current effect. The new user had the same problem.

I don't see much in the systemd logs (journalctl). Sometimes there are messages like:

Apr 16 23:06:51 saizo org.kde.KScreen[11843]: kscreen: Primary output changed from KScreen::Output(Id: 636 , Name: "DVI-I-0" ) ( "DVI-I-0" ) to KScreen::Output(Id: 636 , Name: "DVI-I-0" ) ( "DVI-I-0" )

and

Apr 16 22:34:17 saizo dbus[1169]: [system] Activating via systemd: service name='org.freedesktop.UPower' unit='upower.service'

But it seems sometimes those messages don't appear and the slowdown is still there so I think they aren't relevant.

Here's the .xsession-errors file for the new user, andhere's the .xsession-errors file for my existing user; I didn't see anything obvious.

Anyone have any idea what's going on here? I see other posts here and online about 30 second delays, but no one else seems to have two minute delays.