Further to this, it seems my users are pressing send but then due to whatever reason nothing is happening for a long time other than the browser showing that the page is loading (sometimes minutes). So they go back in the browser to the previous page and send again. Then two messages come through.
Why would it take so long to send? I have about 40,000 messages in the database and things seem to run slower now, is this a limitation on my cheap hosting package or the plugin.
This plugin uses wordpress CPT and use build in wordpress functions to do its job. So it should not have performance impact. BBPress also use CPT and wordpress org use BBPress for support forum. Support forums have millions of topics and replies without performance impact.