When performance mode is enabled, it disables falling coin animations. Multiple falling coins cause frame dips on A500 so this can help.
On Hard mode enemies do more damage. And energy and life potions will not spawn. You don't really feel it's harder. You just run out of hearts sooner.
Some examples:
Patrolling enemy damage:
easy -16.6% energy
medium -25% energy
hard -50% energy
Turret damage:
easy -25% energy
medium -33.3% energy
hard -50% energy
Spikes damage:
easy -25% energy
medium -50% energy
hard -100% energy