Is it bad practice to route high speed signals (like a SPI bus clocked at 4MHz) through PCB vias?

I've noticed a good bit of noise (+-300mV) on my SPI bus signals with 3.3V levels.  The signals traces are only about 5cm long but they go through about 5 vias each on the way to their destination. The board has only 2 layers which is why there are so many vias on these lines.

What kind of noise can I expect (if any) to be introduced by a PCB layer change via?