[dhosek]

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. One thing I found really fascinating is that even though I’d never studied French¹, I actually had an easier time understanding a conversation between my ex-wife and her aunt in French than when they spoke Spanish in which I was functional (my skill in the language has gone up a great deal since then so that I now read fluently, and speak and listen reasonably well, albeit less well than I would like).

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1. Thanks to my kids studying French on Duolingo and my joining them, I can no longer say that I’ve never studied it.

[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.