NiMh are only faster at the beginning of a run and only with certain Motor sizes due to the resistance both internal as well as external that NiMh cells have.
The curve of a NiMH is like an arch going down steeper and steeper from the first second. A LiPo has more of a straight slanted slope so within the first minute or two the NiMH begins to show less voltage then the LiPo.
For some people speed is measured before the first lap so for them it is safe to say a NiMH with 6 cells that tops out at close to 9.6V volts delivers more power and a LiPo that tops out at 8.4 delivers less. However it is not that black and white as LiPo have less internal/external resistance (this is why NiMH get so hot, that is power being wasted) and can deliver that power more freely.
Think of it like a garden hose hooked to a bucket. if both batteries are the same capacity then the LiPo has a bigger hose where the NiMh has a smaller hose giving it more pressure but this creates leaks as it flows which wastes water.