We follow Shopify's best practices to regularly audit the affect of Withfriends on online store speed as we build features.
We found a minor (-3.2%) affect on online store performance for members who are logged in. That's because our app actively monitors their actions to ensure they receive their member discounts.
Other than that, across several tests, we found no effect on other visitors of your site at all.
You can audit our Lighthouse reports here:
All our tests are performed with all Withfriends features turned on (add-to-cart upsells, member badge, post-checkout upsells), therefore, it's safe to use these features without affecting online store speed.