Advanced Graphing vs. plotimplicit()

[parisse]

Make sure you are using x and y lowercase, and enter limits for x and y (otherwise the defaults will be used and this takes forever)
plotimplicit(tan(x)-sec(y),[x=0..pi/2,y=0..pi/2])
This should be run in the CAS history, because in the geometry app it will be recomputed several times, and this is costly.
And of course here, it's easier to express y in terms of x or x in terms of y on each subrectangle of periodicity/symmetry.
plotfunc(acos(1/tan(x)),x=0..pi/2)