When compared to mirrorless cameras, DSLRs by design have some inherent limitations. Part of it has to do with the fact that SLR cameras were initially developed for film. When digital evolved, it was treated just like film and was housed in the same mechanical body. Aside from the circuitry required for a digital sensor and other electronics, new digital film media and the back LCD, the rest of the components pretty much stayed the same. The same mechanical mirror, the same pentaprism / optical viewfinder, the same phase detection system for autofocus operation.
