Doesn't matter where the carer is coming from, when her other clients are, whether she is being paid for travel (some only get mileage) or any other lame excuse that several of you here have come up with, she is scheduled to arrive at X time, and stay for X length of time. If she gets there early, she bides her time in her car until her scheduled time.
If you know one of your carers will be earlier than the others, can't you wait to have breakfast until after her visit, OP? No, she can't and shouldn't have to.
The op, being the customer, is the one who dictates times, not the carer.
If the carer is from a specific company op, call her line manager and tell them this is an ongoing issue. If she is independent, be honest and say she is arriving to early, and it's inconvenient. In future, refuse to let her in until the scheduled time.
FWIW, I work in an environment where I schedule carers days, and the care we offer is consumer directed. In other words, the consumer decides timings etc, and carers would be reprimanded for continuing to turn, and wanting to access the consumer's home early, because as op said, it makes them feel rushed and uncomfortable.