Deb
Do you have an outside the foundation cleanout on the line that goes to the septic tank? They can really aid you in diagnosing problems like this. If there is a problem with the septic tank or line from cleanout to septic, there will be standing water in the pipe. This at least gives you a place to start--is the problem inside or outside the home. It is really a good idea to check your septic tank annually. If the level of sludge gets above the outlet baffle, it will flow out the laterals and can clog everything up. The liquid part of the tank contents drain by gravity to a drainage field where pipes with holes are laid in a gravel bed underground. The liquid percolates away through the gravel bed and down through the soil. Clogging the holes with toilet paper, human waste and anything else that should have stayed in the tank is a major thing and is easily preventable by checking your tank annually and getting it pumped when the sludge level is about 1/3. However, most tanks do not need to be pumped more than every couple of years. So it could be the inlet baffle.
No standing water in the cleanout or above inlet baffle in tank--it could be a clog in the drainage line inside the house. This toilet may well dump 5 gal everytime it flushes but, at the least 3.5 gal.
Or you may have more than one problem here. You could have a partially clogged drainage line and a toilet that is just getting old. As toilets get old (or in bad water conditions or if you use those stupid Clorex cake tablet things in the tank that flake off and clog the holes), the holes around the rim and the large hole in the bottom (not all toilets have these) can get clogged and not allow the water to get from the tank to the bowl fast enough to create a good flush. You can check this by taking a five gal bucket of water and quickly dumping the contents into the toilet bowl. If it "flushes", buy a new toilet and don't even try to clean the holes in that water wasting dinosauer. Do this when the toilet and other bathroom fixtures have not been used for awhile in cse you have a partial drainage line clog too. You may want to consider replacing it even if it is okay. There are very good 1.6 gpf toilets now and those old 5 and 3.5 gpf toilets are real water wasters. Don't know how many people live in your household, but I'll bet you would be surprised how many times a month a toilet is flushed.
Many plumbing problem causes are not readily apparant and take some investigation to narrow down the possibles.
Deb
The Pipewench