SpriteFont, you mean Text plugin in C2, try not to use too many in screen too. Because when i creat a number for health bar, mana bar, items, status, etc... It's cause down the performance, it's force me to use shortcut key to visible text plugin ( and set position ) when hold down. And you should be careful with animation too, i try to make 3-4 character with animation walking on screen, the performance is low down, :)) in my game, my character fighting with 2-3 guys on screen, the fps drop to 32-40 fps . With tips number 1 up there, there is another way, just creat the enemy or NPC using (distance) if you get near, and destroy them if you run faraway from them, it'll bost the performance too.
And about the error, it's never happen with me, you should send the mail for — But have you try to make a zip file and run offline in cocoonjs, there are 2 ways, First you export a zip file and upload to dropbox then copy URL and run it, second is copy zip file to your mobile's sd card, run it offline.