Yeah it'll depend on how good your coreboot implementation is. AFAIK it's pretty good on Chromebooks because Google whereas a corebooted ThinkPad might have some downsides to it.

The slowdowns I would attribute to likely bad power management, because ultimately the code runs on the CPU with no involvement with the BIOS unless you call into it, which should be very little.

Looking up the article seems to confirm:

The main reason it seems for the Dasharo firmware offering lower performance at times was the Core i5 12400 being tested never exceeded a maximum peak frequency of 4.0GHz while the proprietary BIOS successfully hit the 4.4GHz maximum turbo frequency of the i5-12400. Meanwhile the Dasharo firmware never led to the i5-12400 clocking down to 600MHz on all cores as a minimum frequency during idle but there was a ~974MHz.

I'd expect System76 laptops to have a smaller performance gap if any since it's a first-party implementation and it's in their interest for that stuff to work properly. But I don't have coreboot computers so I can't validate, that's all assumptions.

That said for a 5% performance loss, I'd say it counts as viable. My games VM has a similar hit vs native. I've been gaming on Linux well before Proton and Steam and have taken much larger performance hits before just to avoid closing all my work to reboot for break time games.