It's entirely possible! Like Jonathan mentioned, if shops add code to your storefront, then it can easily result in slowdown. Most apps are not going to cause a significant slowdown, but it only takes one bad app to cause a performance issue.
You can diagnose a bad app by disabling each app, and seeing if the site speeds up. If disabling one app speeds things up significantly, then you've found the app that is the cause of the slowdown. As Shopify suggests, it's best to get help directly from the third-party developer responsible for building the app! Contact the app's developer with your concerns, and ask them for help.
On a side-note, I love the concept of ready-to-make craft kits. I will be sure to check out the Easter-themed kits!
We have an industry leading 58ms average response time. This is exceptionally fast thanks to our performance team.
That being said, Anything added to your storefront will increase loading times for visitors: scripts, images, etc. We have no control over assets added by third party apps, nor their response times. Apps are exceptional sources of additional features, but be sure to understand the changes they make on your storefront.
I just attended an SEO/SEM conference called SMX West. They mentioned a markup language and I was wondering if you are going to be including it into Shopify. It was called Good Relations.
Matt Cutts of Google and Duane Forrester of Bing! also stated that Schema.org and Open Graph will be the future. Any plans to build that into the CMS?
It's slightly off topic for this particular thread, but we already include Schema.org definions. For example, our Skeleton Theme includes some definitions by default and is what most new themes are based on.