After a quick investigation I found that this was the cause of it, so I removed it:
<iframe
src="https://secure.mohanjith.com/wp/web-invoice.php"
style="float: right; width: 187px; height: 220px;"></iframe>
Whilst I think your very much entitled to have this in the admin panel of your plugin, when your server becomes unresponsive or very slow, thus slowing me down at my end when updating settings, I had no option but to remove this code.
Should I end up actually using your plugin, I shall happily donate however.
Just thought I'd post this here in case any other users are experiencing the same issue.
Regards