Mirai can't perform better because the fuel cell can't put out enough burst power, it needs a buffer battery as is for acceleration and regenerative braking and the buffer battery isn't big enough either for Tesla like acceleration due to size and weight constraints.
Re:Hydrogen fuel cell cars (2020)
by new_realist on Monday July 05, 2021 @11:20AM
(#27738851)
The Mirai offers more range; itβs not apples to apples. FCEVs do have some constant overheads around the fuel stack weight.
https://www.caranddriver.com/toyota/mirai/specs
https://www.caranddriver.com/tesla/model-3/specs
As far as acceleration, that's just a design parameter, as both are driven by electric motors. I'm surprised the Mirai can't perform better given it's extra weight.