What's your CPU? I have an iPhone that runs this game ok if desktop mode is requested in Safari. If you're on a desktop, and the CPU is performing worse than an iPhone, that's a sign that you're past due for an upgrade.
They could use GPU instead of CPU for the AI. That said, The AI shouldn't need to be big enough to cause performance issues. There is sometimes some lag when you press some of the skill/senses buttons.
