Chegewara,
Thanks for the suggestion. I got rid of my old computers and have been using SSDs for a couple of years now. My data above were an i5-7400 with Samsung 850 PRO SATA SSD (~500MB/s read) vs. an i9-10900K with Samsung 970 EVO PCIe SSD (~3200MB/s read).
The i5 has 4 cores/8 processes with 3.0GHz speed. Good entry-level gaming computer on a good discount.
The i9 has 10 cores/20 processes with 3.7GHz speed. Pretty much the fastest I can get without having to build my own, no discounts.
Comparison after I turned on exception for both machines:
i9-10900K
no -j make clean; make app
4m 5s
with j make -j clean; make -j app
30s
-j10
43s
-j16
35s
-j20
32s
i5-7400
no -j make clean; make app
I really don't want to do this
with j make -j clean; make -j app
1m 39s
-j8
1m 41s
-j4
1m 48s
My conclusion is that just let make do as many jobs in parallel as it can and it works slightly better than telling it to match the CPU's total process count. If you go with total core count, you get worse outcome than going with total process count.
I hope this justifies my spending