On my slow arm (about 50% the speed of a pi) it takes about a minute to load the calendar module. Almost all that time is parsing timezones.
So, limiting the size of the timezone file for slow devices would help.
This is somewhat surprising, becase I can not remember it taking minutes to load the calendar module on a 110Mhz SPARC. And those are slower, albeit not that much, than my slow arm.