Hey there,
It’s frustrating when your site slows down. There are a bunch of possible reasons for this, and they’ll differ for each unique site.
Have you had a look at our documentation?
https://woocommerce.com/posts/woocommerce-site-slow-fixes/
I realize it sounds suspect that you’ve updated to WooCommerce 3.5 and now you’re seeing these issues. It’s likely that it’s more of a combination of things, though, which have surfaced when updating.
If you’d like some initial insights, please share a copy of your site’s System Status? You can find it via WooCommerce > Status.