When b = 'x' is true for many rows and you select * or multiple columns, then it's the opposite, because reading all row data is slower in column based data structures than in row based ones.

IMO it's easier to explain in terms of workload:

- OLTP (T = transactional workloads), row based, for operating on rows - OLAP (A = analytical workloads), column based, for operating on columns (sum/min/max/...)