While I love the functionality of this contact form plugin, I am officially throwing in the towel. With exception to one WordPress website, every time I have installed this plugin on my domains, it slows every site on the same hosting account. Either it takes them forever to load or it results in a 500 Internal Server Error.
My latest two installations both had the problem with the spinning wheel never confirming success once the submit button had been clicked. Since the websites were both new and the only other two things I had installed were the theme and a slideshow plugin, I know it didn't have to do with compatibility issues on those sites. And I certainly can't deactivate every plugin / theme across the other sites on that same hosting account.
What I can tell you is that all my sites on the same hosting account as the ones I installed this plugin on were slow as snails last night and this morning. After I went into FTP and deleted this plugin from both sites, everything is working smoothly again. I've never had this issue with any other plugin, so it's really frustrating to have to give up great features just because it can't be compatible like other free plugins are.
Anyway, I'm posting this in case someone else has slow site speed or 500 Internal Errors after installing the plugin - check this first.