By default the CPU will be limited to a 15W TDP, which means it'll throttle fairly quickly - if you override PL1 to 25W it'll work much better
(Right now the Skylake code in Coreboot doesn't seem to have support for overriding this in the firmware, so I'm using the Linux RAPL driver to do it)
Recently merged? https://review.coreboot.org/cgit/coreboot.git/commit/?id=b54d15487ace81af9e3a1f9f64a509624bb5081d
Oh nice! I should rebase.