Ubuntu 19.04 with MATE up-to-date.
After a recent update, choosing to add the command applet to the panel has very long response time and eventually the entire panel itself becomes unresponsive.
In the journal I see this
command-applet[2437]: gtk_label_set_text: assertion 'GTK_IS_LABEL (label)' failed
command-applet[2437]: g_spawn_command_line_sync: assertion 'command_line != NULL' failed
and
g_set_application_name() called multiple times
and
command-applet[3489]: g_object_set_data: assertion 'G_IS_OBJECT (object)' failed
command-applet[3489]: Failed to get pattern 12583513,1083,14
command-applet[3489]: Failed to get pattern 12583513,787,0
command-applet[3489]: Failed to get pattern 12583513,723,0
command-applet[3489]: Failed to get pattern 12583513,625,0
command-applet[3489]: Failed to get pattern 12583464,1083,14
command-applet[3489]: Failed to get pattern 12583464,787,0
command-applet[3489]: Failed to get pattern 12583464,723,0
command-applet[3489]: Failed to get pattern 12583464,625,0
command-applet[3489]: Failed to get pattern 12583177,787,0
command-applet[3489]: Failed to get pattern 12583177,1083,14
command-applet[3489]: Failed to get pattern 12583177,787,14
command-applet[3489]: Failed to get pattern 12583177,723,0
command-applet[3489]: Failed to get pattern 12583177,625,0
command-applet[3489]: Failed to get pattern 12582980,723,0
command-applet[3489]: Failed to get pattern 12582980,1083,14
command-applet[3489]: Failed to get pattern 12582980,787,14
command-applet[3489]: Failed to get pattern 12582980,723,14
command-applet[3489]: Failed to get pattern 12582980,625,0
command-applet[3489]: Failed to get pattern 12582980,-1,-1
command-applet[3489]: Failed to get pattern 12582980,-1,-1
command-applet[3489]: Failed to get pattern 12582980,-1,-1
And then once in a while:
command-applet[2422]: command-applet: Fatal IO error 11 (Resource temporarily unavailable) on X server :0.
But this is rare and irregular so may be not directly related.
The scripts I use to populate the applet are simple and when run alone complete immediatly.