Do we know what mechanism causes efficiency loss at mismatched RPMs? What if multiple props are arranged in a series, akin to how turbo engines have stages wit

Do we know what mechanism causes efficiency loss at mismatched RPMs? What if multiple props are arranged in a series, akin to how turbo engines have stages with different compression ratios, but instead of a fixed shaft with varying blade geometry we have varying differential rotation speeds between them adjusted based on RPM?