Dolphin has become very slow after recent update

[Bug Reporter]

Dolphin has become slow recently. In particular, adding or moving a
bookmark in the Places panel has a delay of several seconds.

To reproduce, create a new bookmark in Places or move an existing bookmark
up or down in the Places panel. There is a delay of a few seconds.
Previously, there was no noticeable delay. I now experience these delays
100% of the time.

Just now I timed it and there is a delay of about 5-6 seconds before the
moved bookmark appears moved, then there is a delay of another second or so
before Dolphin's GUI is responsive again.

Also, if I leave Dolphin open all day, even normal navigation from location
to location also becomes very slow. To reproduce, navigate to any location
using Dolphin. There is a delay of several seconds before Dolphin refreshes
to show the new location. During this time the Dolphin UI is unresponsive
(Dolphin appears frozen briefly). Previously, there was no noticeable
delay. I only experience this if Dolphin has been open for a while. If I
close and reopen it, I do not get the very long delay when navigating to a
new location.

I'm running KDE on Arch.  Here's some system info:

   - plasmashell --version: plasmashell 5.12.5
   - dolphin --version: dolphin 18.04.0
   - Linux 4.16.6-1-ARCH #1 SMP PREEMPT Mon Apr 30 12:30:03 UTC 2018 x86_64
   GNU/Linux

The previous time I looked at my version, I had plasmashell 5.12.3 (a
couple months ago). Dolphin was fine in that version. So the problem
started after that version, but it may have appeared before my current
version. I first noticed it several weeks ago. Nothing about my system has
changed other than normal Arch updates.
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