Googling, I found some similar bug reports: https://bbs.archlinux.org/viewtopic.php?id=243737 https://forum.level1techs.com/t/gtk-applications-are-super-slow-at-startup/166540
The Arch discussion is very similar, as we are both using i3 launched with startx. The Arch discussion pointed to their wiki page on xinit, which includes this note: ------ Note: At the very least, ensure that the last if block in /etc/X11/xinit/xinitrc is present in your ~/.xinitrc file to ensure that the scripts in /etc/X11/xinit/xinitrc.d are sourced. ------ I did that, and it fixed the problem. The file /etc/X11/xinit/xinitrc on my Debian system merely sources /etc/X11/Xsession, which is attached for your reference. It does a lot of things and references other files... I have no idea what actually fixed my problem.
#!/bin/sh # # /etc/X11/Xsession # # global Xsession file -- used by display managers and xinit (startx) # $Id: Xsession 967 2005-12-27 07:20:55Z dnusinow $ set -e PROGNAME=Xsession message () { # pretty-print messages of arbitrary length; use xmessage if it # is available and $DISPLAY is set MESSAGE="$PROGNAME: $*" echo "$MESSAGE" | fold -s -w ${COLUMNS:-80} >&2 if [ -n "$DISPLAY" ] && which xmessage > /dev/null 2>&1; then echo "$MESSAGE" | fold -s -w ${COLUMNS:-80} | xmessage -center -file - fi } message_nonl () { # pretty-print messages of arbitrary length (no trailing newline); use # xmessage if it is available and $DISPLAY is set MESSAGE="$PROGNAME: $*" echo -n "$MESSAGE" | fold -s -w ${COLUMNS:-80} >&2; if [ -n "$DISPLAY" ] && which xmessage > /dev/null 2>&1; then echo -n "$MESSAGE" | fold -s -w ${COLUMNS:-80} | xmessage -center -file - fi } errormsg () { # exit script with error message "$*" exit 1 } internal_errormsg () { # exit script with error; essentially a "THIS SHOULD NEVER HAPPEN" message # One big call to message() for the sake of xmessage; if we had two then # the user would have dismissed the error we want reported before seeing the # request to report it. errormsg "$*" \ "Please report the installed version of the \"x11-common\"" \ "package and the complete text of this error message to" \ "<debia...@lists.debian.org>." } # initialize variables for use by all session scripts OPTIONFILE=/etc/X11/Xsession.options SYSRESOURCES=/etc/X11/Xresources USRRESOURCES=$HOME/.Xresources SYSSESSIONDIR=/etc/X11/Xsession.d USERXSESSION=$HOME/.xsession USERXSESSIONRC=$HOME/.xsessionrc ALTUSERXSESSION=$HOME/.Xsession ERRFILE=$HOME/.xsession-errors # attempt to create an error file; abort if we cannot if (umask 077 && touch "$ERRFILE") 2> /dev/null && [ -w "$ERRFILE" ] && [ ! -L "$ERRFILE" ]; then chmod 600 "$ERRFILE" elif ERRFILE=$(tempfile 2> /dev/null); then if ! ln -sf "$ERRFILE" "${TMPDIR:=/tmp}/xsession-$USER"; then message "warning: unable to symlink \"$TMPDIR/xsession-$USER\" to" \ "\"$ERRFILE\"; look for session log/errors in" \ "\"$TMPDIR/xsession-$USER\"." fi else errormsg "unable to create X session log/error file; aborting." fi exec >>"$ERRFILE" 2>&1 echo "$PROGNAME: X session started for $LOGNAME at $(date)" # sanity check; is our session script directory present? if [ ! -d "$SYSSESSIONDIR" ]; then errormsg "no \"$SYSSESSIONDIR\" directory found; aborting." fi # Attempt to create a file of non-zero length in /tmp; a full filesystem can # cause mysterious X session failures. We do not use touch, :, or test -w # because they won't actually create a file with contents. We also let standard # error from tempfile and echo go to the error file to aid the user in # determining what went wrong. WRITE_TEST=$(tempfile) if ! echo "*" >>"$WRITE_TEST"; then message "warning: unable to write to ${WRITE_TEST%/*}; X session may exit" \ "with an error" fi rm -f "$WRITE_TEST" # use run-parts to source every file in the session directory; we source # instead of executing so that the variables and functions defined above # are available to the scripts, and so that they can pass variables to each # other SESSIONFILES=$(run-parts --list $SYSSESSIONDIR) if [ -n "$SESSIONFILES" ]; then set +e for SESSIONFILE in $SESSIONFILES; do . $SESSIONFILE done set -e fi exit 0 # vim:set ai et sts=2 sw=2 tw=80: