If all the drains in the house are slow, you may be getting root intrusion into your septic lines. Bath/shower lines do not normally clog past the S trap. If it's hair, one of the more effective ways is to snake a wire coat hanger down the line with a hook bent on the end.. it can be pretty icky though. I've also found a wet shop vac can remove clogs from the S traps if there is standing water. This is esp handy if sand is causing the blockage. With root intrusion, there are chems that are made for killing the roots but it can take time for the blockage to rot away.
Cheap soap can cause a build up on the inside of the pipes that can be esp difficult to remove. It takes many years for the build up to happen but then you need a grinder on the end of a line hooked up to a drill.
You can check to see if it's past the S traps by dumping about 10 gallons of water down the toilet. If you start getting water coming up through the shower, you have some serious problems. Most of the time, there is enough clear pipe in the toilet for the 2 gallons that go down per flush without causing backups.