The first tile floor I did in an apartment I followed everything by the book, took 4 hours to rip up tjhe old linoleum, because I was worried about the "flex" and followed the exact instructions I was told. Waaaaay too much time and effort.

Now, if the linoleum, vinyl squares, etc can't be ripped up easily I concnetrate on eliminating all the flex in the floor befor ethe start of the job. I obsessively walk back and forth on the floor, jump up and down, mark all squeaks, figure out where the joists are, and set out to eliminate that flex. Sometimes 10 screws will do the job in well placed spots, sometimes you need 40. The key is if the subfloor has been subjected to floods over the years, nd if it has lost any of its strength.

If so, then you need to shore up key areas, or rip up the floor in spots and sister 2x4s to the joists. PITA, but a necessary step if its bad enough.

In most cases this isn't needed, so I eliminate the squeaks, and lay hardibacker right over the immovable top surface, as long as it's level and solid.

Set up with the hardi screws where indicated on the templates, make sure no screw heads are sticking up, lay my underlayment patch on the seams. You can do this in the morning and be cutting and fitting your tile in the afternoon. It may not be what the books say, but I've never had a cracked tile on an install, or had one crack in the years afterward.

We're talking about middle class houses and rentals here, anything high end you would rip up the floor if needed.

Git er done, progress not perfection.