there are a couple of things you can do to improve resuming on WP8.
First of all you must merge this change in your local MonoGame source if you don't want your game to crash after a couple of resumes. It will improve the speed somewhat since it doesn't create new buffers,etc but not running out of memory is what's important here.
Then you can merge this which will greatly improve loading speed.
If you have compressed assets from the Monogame pipeline, use this one too. It will avoid one extra memory copy,
And finally, tell ContentManager which assets need reloading and which don't, avoid to reload/recreate things like SoundEffects etc.