Just installed a letsencrypt certificate on my siteground domain theemeraldemporium.co using the automated option in cpanel.
Attempting to go to my frontpage (https://theemeraldemporium.co/) is VERY slow and ultimatly results in a warning message:
This server could not prove that it is theemeraldemporium.co; its security certificate is from *.sgcpanel.com. This may be caused by a misconfiguration or an attacker intercepting your connection.
The shared SSL is installed globally on the server and all clients can use it. Its certificate is self-signed and is configured to work with SiteGround server's name. That's why it will always show a pop-up warning when used.
The pop-up warning seems to have morphed into a complete block, at least on chrome.
Does this suggest that a letsencrypt certificate on a shared SSL account is in fact pretty useless?
No, this is an issue with the site’s SSL/TLS configuration, not a problem that affects shared hosting plans in general. I don’t know anything about SiteGround, unfortunately, so I don’t really know where you should start looking. I’d recommend contacting their support.