@User i tried simulating a 7-stage ring oscillator with AD/AS/PD/PS areas/perimeters as calculated by xschem and removing all these parameters and specifying only W, L, nf=1: ring oscillator period reduces from 673ps to 623ps: The relative difference will be even higher if i remove the parasitic capacitances i have in this test schematic.The inverter without area/perimeters is:

.subckt lvtnot  a y  VCCPIN  VSSPIN      W_N=1 L_N=0.15 W_P=2 L_P=0.35
XM2 y a VCCPIN VCCPIN sky130_fd_pr__pfet_01v8_lvt L=L_P W=W_P nf=1
XM1 y a VSSPIN VSSPIN sky130_fd_pr__nfet_01v8_lvt L=L_N W=W_N nf=1
.ends

while the inverter with area/perimeters specified is:

.subckt lvtnot  a y  VCCPIN  VSSPIN      W_N=1 L_N=0.15 W_P=2 L_P=0.35
*.opin y
*.ipin a
XM2 y a VCCPIN VCCPIN sky130_fd_pr__pfet_01v8_lvt L=L_P W=W_P nf=1 ad='int((nf+1)/2) * W/nf * 0.29' as='int((nf+2)/2) * W/nf * 0.29'
+ pd='2*int((nf+1)/2) * (W/nf + 0.29)' ps='2*int((nf+2)/2) * (W/nf + 0.29)' nrd='0.29 / W' nrs='0.29 / W'
+ sa=0 sb=0 sd=0 mult=1 m=1 
XM1 y a VSSPIN VSSPIN sky130_fd_pr__nfet_01v8_lvt L=L_N W=W_N nf=1 ad='int((nf+1)/2) * W/nf * 0.29' as='int((nf+2)/2) * W/nf * 0.29'
+ pd='2*int((nf+1)/2) * (W/nf + 0.29)' ps='2*int((nf+2)/2) * (W/nf + 0.29)' nrd='0.29 / W' nrs='0.29 / W'
+ sa=0 sb=0 sd=0 mult=1 m=1 
.ends