@User the 2 minutes are mostly the time for ngspice to read the spice models, the inverter simulations takes just some milliseconds. To speed up ensure you have a

.spiceinit

file into the directory where ngspice is working (usually ~/.xschem/simulations) with following content:

set ngbehavior=hsa
set ng_nomodcheck

If you want to further speed up by 4x use this element to load only one corner:

sky130_fd_pr/corner.sym

Unfortunately ngspice spends considerable time loading unused process corners, something I don't really understand why, but we have to live with that.