We can’t speak for all apps, but using Quantity Breaks & Discounts won’t slow down how fast your online store operates. If you were facing any issues with your website in correlation to using the app, you will have access to 24/7 support from a service engineer to resolve the issue.  When choosing an app from the Shopify app store, read the customer reviews. This is a telling way to understand how an app really works, and will likely mention if website speeds have been impacted because of the app if a common issue.