If the shower, kitchen sink, and guest toilet are all slow at once, the issue is likely downstream near the tank.
In a healthy tank, solids (sludge) settle to the bottom, while scum (oils and grease) floats to the top. The middle layer—liquid effluent—exits the tank. Over time, if the tank isn't pumped every three to five years, the sludge layer grows upward. Eventually, it reaches the outlet pipe, blocking the flow of liquid. The system backs up, and the house effectively becomes a plugged bottle. clogged septic tank