Well, cpu is a complex world. It depends a lot on which library do you use, how well optimized is it, which cpu model do you have… Think that not all the cpus can execute same basic operations.
Toy example, supose you have a cpu which can sum but not multiply. It will need lot of cycles to perform a multiplication meanwhile if cpu has an specific module enabled will be much faster.
In fact there is one specific operation very important for deep learning whose name I forgot. It’s a multiplication + sum.
What exactly did you find?