I am using Experience Builder Online version. I find the performance of the hosted web application is very poor. (not sure if the Experience builder or the hosting website is to blame ) : I have a very simple webapp with only one map widget and two web maps, but it takes 14 seconds to load on my phone. My phone is Pixel 4XL, which is not a low-specs phone.
When I load the same webapp on my desktop, it is not much better in regard to the performance either...
Do you have the same issue? Are there any templates tailored for mobile phone?