To test the behavior of newly enabled gnome extensions and how they might affect your CPU usage without needing to log out, you can run a new gnome-shell nested(in a window) like so:
dbus-run-session gnome-shell --nested
You can set the window size with the environment variable MUTTER_DEBUG_DUMMY_MODE_SPECS like so(e.g. window size 1636x800):
MUTTER_DEBUG_DUMMY_MODE_SPECS=1636x800
and use it like so:
MUTTER_DEBUG_DUMMY_MODE_SPECS=1636x800 dbus-run-session gnome-shell --nested
Extra information available at man gnome-shell and man dbus-run-session
The output from that session will also be printed in the parent terminal window for tracing/debugging.
On the other hand(a bit more advanced though), you can use strace to trace what system calls/signals are being used by/for the running gnome-shell process/s and e.g. print a 10 second summary like so:
for i in $(pgrep -f "gnome-shell")
do
    echo "Tracing process: $i"
    sudo timeout 10 strace -c -p "$i" # administrator privileged account needed for strace to attache to running services
done
You can also attach to and debug running processes using other methods ... Some of which are discussed in this answer.