Basemap +o: the smaller the slower

I have discovered that basemap +o might be fine for maps of
all 50 States, but when one zooms in, slowness mounts up:

#n=42 s=32 w=-88 e=-78 #quick
#n=42 s=41 w=-88 e=-87 #slow
n=42.3 s=42.2 w=-88 e=-87.9 #so slow I never did wait to find out
set --  $(gmt vector -A$w/$s -Tp-57)
gmt begin zzz
gmt basemap -R$w/$e/$s/$n -JM1i -Bafg -B+o$1/$2
gmt end show