Buses tend to have their drawbacks too, tbf.

For one, they're less scalable - to increase capacity on a busway, you need more buses, requiring more drivers and more infrastructure including passing lanes. In Brisbane, where issues in this area have resulted in service cancellations, the latter can be crucial. Buses tend to be less efficient owing to rolling resistance - nothing beats steel-on-steel and a pool of motors. Flexibility can be a liability too - for example, service cuts and reductions in service quality are easier to perform with buses. This carries over to infrastructure projects too - for example, the Northern Transitway bus lanes have seen cuts into a rush-hour only system, decreasing effectiveness as a result. And while the light rail is more expensive w.r.t initial investment, the operating costs would be considerably lower. It's a case of "spend money to make money", to save in ongoing things like operating costs and energy usage. That said, busways do have some advantages, including insanely high theoretical throughputs (which do come at the costs of higher operating expenses and a need for more vehicles). I am not exactly super clued in to address the speed aspect, though I will argue that speed is merely one factor in service attractiveness (frequency, comfort, permanent infrastructure, separation from road traffic, etc are aspects too, which I argue G:link does well from my rides on the system).

In the end, it all comes down to the needs of the corridor. Buses, trams, and trains all have their niches, which they each fulfill well in certain contexts. For the corridor, a linear, populous area with lots of patronage and a need for a permanent right-of-way with good presence in the urban area, I'd argue that the light rail does the job well. Brisbane's busways, which were designed to service low-density car-centric suburbia in a commuter role, do well in that environment too. But that's my 5 cents