MCP:

A toilet is really nothing more than a glorified syphon. In a regular syphon hose, you suck on the end of the hose to fill it with liquid, and as long as the downstream end of the syphon hose is at a lower elevation that the liquid in the tank you're syphoning from, the Laws of Syphon Physics take over to keep the liquid flowing through the syphon hose.

No one would buy a toilet that you had to suck on the outlet end to get it to flush, so instead what happens is that water from the toilet tank flows into the bowl in sufficient quantity and fast enough to completely fill the channel leading from the bowl to the outlet of the toilet. Once that channel is full of water, those same Laws of Syphon Physics take over and turn that discharge channel into a big 2 1/2 inch diameter syphon hose that sucks the bowl dry. So, by filling that discharge channel full of water somehow, it transforms into a giant siphon hose, and the flushing of the toilet bowl is nothing more than that giant siphon hose sucking the contents out of the bowl.

So, what you should do is pour a 5 gallon pail of water into each of your sluggish toilets as quickly as you can without spilling water all over the floor, and observe each toilet bowl to see that it not only flushes, but flushes with enthusiasm! If any of your toilets flush better under the 5 gallon test, then the problem is upstream of the water in the bowl in those toilets. It could be that the toilet tanks don't have enough water in them, or that the tank water isn't flowing into the bowl fast enough. And, that could be due to the flush valve not opening fully or that the holes under the rims of the bowls or the jet hole at the bottom of the bowls are clogged up and not allowing water to flow through them, and therefore reducing the rate at which water flows into the bowl.

If a toilet is still sluggish with the 5 gallon pail test, then the problem with that toilet is downstream of the water in the toilet bowl. It could be that your drain piping is partially clogged with solids (mostly from the kitchen sink), and what you need to do is have the main drain line from your house cleared.

So, do the 5 gallon pail test on each toilet and let us know what the results are. If you find that you have to stop pouring water into any toilet because the water is about to overflow the bowl, then that's almost certainly the result of your main drain line being clogged up with solids from your kitchen sink, and you need to hire a plumber to clear that line with a motorized snake. The "snakes" sold in hardware stores for clearing drains simply aren't long enough or powerful enough to clear a long length of 4 inch drain piping all the way out to the middle of the street or avenue your house is on.