Nouveau was never most performant driver, but developers still been able to achieve something like 50-80% performance on 6xx 7xx series cards before Nvidia started to signed firmware shenanigans.
It's not like I defending the driver performance, but considering for decades it was purely reverse engineering effort by skeleton team < 10 people their achievements are outstanding.
It depended a LOT on the workload.
valley? 20%
glmark2? 91%
vkmark? 11%
To some degree this is probably just the vulkan driver and compiler being really, really immature. I expect that the gap will become a LOT closer in the next 6 months, or whenever they get past implementing vulkan 1.3 in the compiler and can put some work into cleanup.
It's just not currently very fast, yet.