Slow compared to what? All 1050s are 128 bit, that refers the memory bus.
If you're asking if the 128 bit bus is why it performs at the level it does, that's part of the reason but not the entire reason. There are factors such as clock speeds and CUDA cores that also influence performance. This why you might find a videocard with a 192 bit bus that is slower than a videocard with a 128 bit bus.