Sorry to hear about the rejection of your app.
This is actually the first time I've heard of a Mag+ app getting rejected for this reason. The ability to selecting text has not been an issue before. And there are many apps out there on made on other platforms where you are not able to select text. So maybe that's not what you should focus on in this rejection note. The ability to add links for maps, phone and email is also mentioned which is possible to include in your design: http://support.magplus.com/entries/22378023
Regarding the app not being fast or responsive it may be the case depending on what content that is included in the app. Especially when using HTML and embedding videos there are a couple of things you can check in order to have the app load the content faster:
Best size of video: http://support.magplus.com/entries/20947006
Including HTML: http://support.magplus.com/entries/22378023
If I use High res pictures the App become slow and less responsive, to use low res images is not fair.
You should be able to use high res images, the plugin will downsize the images to make sure they are in the correct resolution for the device. What could be causing long loading times could be HTML assets that has not been downsized correctly.
_What should I do now to make the App approved by Apple?
_Depending on the content of the app, Apple might think that this app is more appropriate to be distributed through iBooks. This is if the content is static and is not using any "special" features. When it comes to Mag+ there are several features that can be used such as pinning, popups, dual-layouts, linking within our outside the app, embedding videos etc.
The UI of the App is not editable, is Mag+ design so why Apple is rejecting it?
The rejection in this matter seems to be based on the performance of the app when the MIB is presented. The graphical assets for the UI is fully editable but there have never been a rejection for the UI of the app.
Maybe I should convert my App into a book for the bookstore?
As mentioned above, if Apple think that the content is more appropiate to be available through iBooks then that might be the case. In order to have the app being distributed as a regular app you can point to features and functions that you are using that is not available through iBooks
Sorry for the late reply! Probably won't do you any good now as this was a while back... Hopefully it all went well!