Deciding when you try the harder pass can be difficult, but for me 30mph didnt start to seem "slow" and "easy" until I started making attempts at 32 and 34.
I would try 1 mph increases, and I would start weaving them in after you complete at least 1 full pass and you have made 3 attempts.
This way your body has warmed up and you get a feel for rythm before attempting faster speeds