Plumbing – toilet slow to flush with bubbles from the jet

There is a clog downstream from both of these drains. If they are both at the lowest level of the house and you have a septic system, then it's conceivable that the problem is there, but you'd also see water backing up into that bathroom when you run any other showers/toilets in the house.

Most likely, the clog is in the joint in the drain lines where the shower and toilet join on their way out of the bathroom. Get a snake to fish down the drain (probably best to do this in the shower to avoid damaging the toilet bowl) and expect to go down quite a few feet before you reach the problem.

Edit: Revisiting this question, I'd agree that there's also a likely vent blockage from the bubbling descriptions. But the fact that toilet water backs up into the shower points to a drain blockage, too, and that will be easier to fix.

I picked up some CLR and scrubbed all the jets under the rim. I even ran some through the tank - open flapper so as to minimize contact with the rubber and plastic. Using a mirror and a small piece of wire I was able to clean out a few stubborn holes.

All toilets were able to take a steady volume of water from a bucket without issue. This lead me to do a little more research. What I found was the hole at the bottom of the toilet that forces water down the drain is actually more susceptible to clogging and plays a larger role in the syphoning action. All three toilets had significant mineral build-up in this hole. A few minutes with my finger was able to clear out quite a lot. Some careful work with a small l-shaped alan wrench brought me the rest of the way home. Whammy!

The two younger toilets now flush like a champ. The oldest was so badly plugged I'll be needing a replacement. I'm gonna try a little soaking with CLR to see if it'll free up but I'm expecting a trip to the hardware store.