Is it possible the fan is failing, and not always spinning up on boot, resulting in the CPU thermally throttling itself? There could also be something wonky with the BIOS, where every so often it initially doesn't spin the fan (or spin it above minimum) and so the CPU only gets cooled when the Windows power management stuff takes over.
Similarly, there might be something wonky in the BIOS power management that doesn't allow the CPU to ramp up to its non-idle frequency until Windows kicks it in the ass. I haven't worried about this kind of thing in years, but completely resetting the BIOS might help here.
The fan definitely works and is not making any noises etc. Speccy reports fan speeds and you can see it ramp up and down.
Laptop runs cool to warm most of the time even when running most games.
Much cooler than the old Lenovo t420 which would heat soak and then throttle very often.
I'll check the bios again for anything I might have missed.
thanks for your help guys! Googling this issue gave me almost zero hits.
Main machine: Core I7 -2600K @ 4.0Ghz / 16 gig ram / Radeon RX 580 8gb / 500gb toshiba ssd / 5tb hd
Old machine: Core 2 quad Q6600 @ 3ghz / 8 gig ram / Radeon 7870 / 240 gb PNY ssd / 1tb HD