I have a hayward 2 speed self priming pump to be used on a inground spa. My question is will the pump prime at low speed (filtration cycle) or should I use a check valve? Thank you!

"Self-Priming" doesn't always mean the pump will suck water when it's dry.

It all depends on where the pump is mounted in relation to the water level.

If your pump is more than a foot or two above the water level, then check valves are a great help... although they will reduce the flow rate a bit.