performance - What is the proper use of https in a modern E-Commerce environment? in WordPress

HTTPS has an overhead on processing power on every request - security considerations or usage pattern left aside. That's the major downturn (and that it ever was, a question of resources), so you should really ask yourself what do you want to offer to your users.

It's absolutely valid to SSL everything (tm) for your site. That's very user-friendly indeed.

So if your see for yourself as in 3), that performance ist an issue any longer, go for it! I would rate it as the major downturn, but that's mostly related to resources and traffic, so if performance ist an issue, offer HTTPS.

User love TLS, it's a very well accepted principle: secure sites.