I found that in my Ubuntu 18.04, nautilus takes a long time to start. Here is the trace of running it on the command line:
$ time nautilus
Initializing nautilus-dropbox 2015.10.28
Error creating proxy: Error calling StartServiceByName for org.gtk.vfs.UDisks2VolumeMonitor: Timeout was reached (g-io-error-quark, 24)
real 0m40.316s
user 0m0.765s
sys 0m0.141s
I hope it might provide some clues to experts.
I also notice nautilus also causing other apps to start slow, such as gedit and chrome. I tried removing gvfs as well as nautilus will make starting other apps such chrome much faster. But I need a file manager.