You made a mix of things there that caused a misundersntanding with
@notion900 Each one of you is talking about different things. I'll try to explain it all.
Walking around would upset people in any Spanish church. In places like Santiago's Cathedral they may be resigned to it but it doesn't mean there isn't people upset by it.
Arriving a bit late (before the liturgy of the Word) is tolerated and it isn't too rare (i.e.: it happens in many masses that there's people that arrives a bit late). You just need to be quiet and discreet (e.g.: stay at a pew close to the entering door and not go all around the church making noise with your shoes to seat in the most far away pew).
Arriving at the liturgy of the Word might be embarrasing (the later, the more embarrasing) and not everybody is fine with it (i.e.: someone might be upset while others wouldn't be upset).
No one is expected to arrive after the liturgy of the Word except by mistake (e.g.: people that got wrong the starting time of the mass, people that go for other reasons and didn't know there was a mass...).
Leaving at communion time is tolerated and it isn't too rare either (i.e.: it happens in many masses that there's people that leaves at communion time).
If there was some chant right after mass (e.g.: the Salve Regina) or a novena or, in Christmas, the kissing of Child Jesus or.., it's tolerated too to leave when the mass is ended without waiting till the chant or the novena or the kissing of Child Jesus or... is ended. On Maundy Thursday and Good Friday, if you don't leave at
communion time, you are expected to leave after the priest left to the sacristy.
Leaving before communion time is unexpected and very odd. If you had to make it for any reason be as discreet as possible to avoid having all the parishioners looking at you.
So, as
@Anemone del Camino said, coming and going during service (meaning people come and go whenever they want and/or there's continuily people comming and going) isn't accepted and, as
@notion900 said, to arrive (a bit) late and to leave at communion time isn't frowned on (and, as I said above, it isn't too rare).
P.S.: That's the common usage. Personal points of view of some priests may be different what means from time to time you hear at mass priests recalling the importance of attending all the mass (usually on days when too many people arrived late) but that's rare.