When does it make sense to send more inquiries asking about review progress to PLoS ONE? My paper has been under review for 2.5 months now at PLoS ONE. I am still waiting for the first review result.

I have sent three inquiries to ask about the progress, and all the responses were telling me that the paper is out for review and PLoS ONE is doing their best to process my paper efficiently.

In the title of the PLoS ONE website, it writes "accelerating the publication of peer-reviewed science". I am really disappointed. PLoS ONE is the slowest among all the journals to which I have submitted.

My question is, does it still make sense to send additional inquires about the progress to the journal? If it does, what should I write in the inquiry?

===update===

Three months now from the submission, I still have not got the first decision. My case is an example showing Plos One is very slow. If you want to publish something quickly, Plos One is the wrong choice! Indeed, it is the slowest among all the journals I have submitted to.

===update===

Just got the first decision a few days ago. i.e. The first decision is made around 3 months after submission. The review results are very simple and positive actually, simply asking to cite more references and add some discussions. Still can not understand though why so simple and straightforward review comments need 3 months to conceive!
PLOS One are pretty good at hassling editors to get a move on (I know, I am one) and if an editor doesn't move on a paper, they will reassign it to a new editor. So I don't think asking what's happening will help.

The usual reason for delays is reviewers. You ask someone to review. They don't reply. They are reminded. After two weeks, you give up and try to find a new reviewer. You invite them to review. After one week they agree. Two weeks later  a reminder is sent. A week after that another reminder is sent. A week after that you give up and invite a new reviewer. We are at two months, and we don't have a reviewer yet. 

The one thing that you might do to help is suggest reviewers. You might be aware of people who are working in the area that the editor may not know about (for example, because you have seen them present at a conference).