[Muromec]

>Generalizing from my own experience, it’s easier for me to understand a non-native Spanish speaker than a native Spanish speaker and I would guess that the same applies with ESL speakers.

You guessed right -- it's /usually/ easier to understand other non-native speakers, both because of accent and less idioms. That is unless the accent is really heavy and doesn't match your own.

[jgwil2]

I don't think it's really to do with accent, but rather that non-native speakers tend to talk slower and use simpler grammar and a more limited vocabulary (including fewer idioms).

[dhosek]

That does help, I’m sure. There’s also a tendency to incorrectly apply foreign grammatical structures to the language in a way that might be strange or incomprehensible to a native speaker while making perfect sense to a fellow second-language speaker.