`import webbrowser` has heavy side effects. It searches a number of executables. On X Window it also runs external program xdg-settings.

Cold start:

$ time ./python -c ''

real	0m1.719s
user	0m0.088s
sys	0m0.036s

$ time ./python -c 'import webbrowser'

real	0m5.713s
user	0m0.308s
sys	0m0.196s

Hot start:

$ time ./python -c ''

real	0m0.094s
user	0m0.072s
sys	0m0.020s

$ time ./python -c 'import webbrowser'

real	0m1.026s
user	0m0.284s
sys	0m0.100s