I tested this on my own system somewhat recently, with a Ryzen 5950X, 64 GB of 3600 MHz CL 18 RAM and a 1TB Samsung 970 Evo, using the config file that ships with Fedora 33.

I created a ramdisk as follows:

    ~$ sudo mount -t tmpfs -o size=32g tmpfs ~/ramdisk/
    ~$ cp -r Downloads/linux-5.14-rc3 ramdisk/
    ~/ramdisk$ cp /boot/config-5.13.5-100.fc33.x86_64 linux-5.14-rc3/.config
    ~/ramdisk$ cd linux-5.14-rc3/
    ~/ramdisk/linux-5.14-rc3$ time make -j 32

My compiler invocation was:

    ~/ramdisk/linux-5.14-rc3$ time make -j 32

And got the following results

    Kernel: arch/x86/boot/bzImage is ready  (#3)

    real 6m2.575s
    user 143m42.402s
    sys  21m8.122s

When I compiled straight from the SSD I got a surprisingly similar number:

    Kernel: arch/x86/boot/bzImage is ready  (#1)

    real 6m23.194s
    user 154m24.760s
    sys  23m26.304s

I drew the conclusion that for compiling Linux, NVMe might as well be RAM, though if I did something wrong I'd be happy to hear about it!