I know it's supposed to be better to lift with bent legs. But it's easier and faster to keep my legs straight. Is lifting with straight legs really so bad?
Yes. When your knees are straight, your lower back carries most of the stress. When you bend your knees and lift with your legs, your hip and thigh muscles do most of the work.
You are right that lifting with straight legs is in some ways easier. It doesn't take as much energy, and your heartbeat doesn't rise as much. But to help keep your back healthy, you need to lift with your legs. It's especially important if the load is big, but it's also important when you're lifting something small.
Even if you are strong and in great shape, unsafe lifting postures can cause small injuries to the soft tissues of the spine. These "microtraumas" can eventually add up to one big back problem.