I've used Big Commerce, Shopify and WooCommerce across a few clients over the years -- in my opinion, Shopify is the way to go unless you need lots of customizations. Big Commerce's template system is not as intuitive as Shopify (liquid) and the third-party apps are of lower quality too as well. There is a ton of content out there on Shopify and lots of third party support (as well as for WooCommerce). Speed wise, I think Shopify is a good platform. My ecommerce client is getting 70+ scores on PageSpeed Insights which we could probably cut if desired by removing some product images on the homepage if desired.
