My 7 y/o DD went through the Primer when she was 4 and 5. We didn’t do math everyday when she was in preschool, and when she went off to Kindergarten she did a little at home leisurely. She was learning more from the Primer in addition (what is also taught in Alpha) than she was at school from Saxon.
She finished up the Primer when we started first grade at home. When we started Alpha there was so much review that we just skipped those chapters. I had her take the tests, and then we moved on.
With that said, I am doing the same thing with my DS5. He is not writing yet, and Alpha starts out expecting the student to be able to write their numbers. Yes, the first lessons are easy peasy, but this allows the child to also master something confindently while learning new things as they get into place value and addition. Also, I like that the Primer breaks up place value over two lessons. Alpha goes straight into placing up to the hundreds.
My 5 y/o niece is doing Alpha. They are just having to take it slower.