It really depends. I know web sites that are super snappy and apps that are dog slow.

And I would wager the minimum possible startup time is actually smaller for a web page than an app. Apps usually take at least a second or two to open for me, and Google has their page load time down below a second, right?

Either way, your choice of platform is not going to be the thing that prevents you from building a fast app. It's going to be your failure to use the architecture well.