The problem is the sprite font of the size in your project has 30,000 characters, meaning every frame must render 30,000 quads, which is far beyond the budget of most mobile devices.
I've optimised the Sprite Font plugin for the next build so it only renders the on-screen characters. This substantially boosts performance for your given project, but won't do much to help if it's slow with the entire sprite font on-screen. (On iOS Safari this is probably solely down to lack of WebGL support which is supported in iOS 8.)
Sprite Font+ obviously isn't fixed because it's a third party plugin, and this is one of the disadvantages of using unofficial plugins, but if you ask the dev then they may be able to diff the runtime scripts and add the optimisation to the third party plugin as well.
I'd still recommend splitting the object up in to smaller paragraph-sized objects if at all possible, regardless of whether you use sprite fonts or text objects. This allows the engine to quickly skip entire offscreen objects without having to determine whether individual lines/characters are within the viewport.