What are the trade-offs between high-Vt and low-Vt cells?

Main trade-off is between power and speed. High-Vt cells reduce leakage power, which is good for saving energy, but they make the circuit slower. On the other hand, low-Vt cells switch faster, but they leak more current, which increases power consumption.

From my experience, you use high-Vt cells when you want low leakage, like in standby or non-critical paths. But if you and I are designing timing-critical paths, we must go for low-Vt cells to meet speed requirements, even if power goes up.