Why are problems in P easy and other problems not
Your definition of "easy" doesn't make much sense. What exactly does "solving more instances at each time unit" mean?

Usually our notion of hardness refers to our ability to "solve" one instance. More formally, let $T(n)$ be the amount of operations required (proportional to time) for inputs of size $n$, then by "hard" we usually mean that $T$ grows too fast.

As for why polynomial $T(n)$ is considered easy, see this question.