Two things to think about with SSL.
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Google looks at is a trust signal. It shows you are actively trying to protect the users that come to the site. Google had an article on this in 2014. https://webmasters.googleblog.com/2014/08/https-as-ranking-signal.html
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How users perceive the site. I personally take less advice from sites with no SSL. Mostly because, to me, it's a sign they aren't up to date. But also, it can deter certain people from making purchases and subscribing to your list without it.
There is "technically" a slowing down of the site with SSL, but with proper installation and configuration it's not much. There is encrypting/decrypting overhead when the site uses SSL. The word "slower" doesn't mean snails pace. It is usually a few milliseconds difference if configured correctly.
Simply adding an SSL cert isn't enough. It is technically a site change and you'll need to do some things to inform Google that the site has change. I HIGHLY RECOMMEND doing the upgrade to HTTPS during a time the site doesn't have much traffic. It can take a few weeks to implement correctly in the SERPs. You can read more about it here. https://support.google.com/webmasters/answer/6033049?hl=en