Most reviewers and users report that Z7'AF is significantly slower than its DSLR counterpart D850 in continuous mode-H & and AF-C.
First of all, ML' AF and DSLR' AF is very different, with both systems having strengths and weaknesses.
Making Z7' AF in AF-C & CM faster can be achieved while lowering fps. There is a relation between Z7' AF speed in continuous mode and the buffer.
Also the difference in file size and related buffer might result in an advantage in AF speed in certain circumstances for Z6 vs Z7.
Next to a smaller buffer another reason for the difference between D850' and Z' AF is that D850 has 3 operating modules, while Zseries have one.