Low risk
Grease, soap scum and food scraps build up in the trap and waste pipe. If the U-bend is clear and it is still slow, the blockage is further along or in the drain.
Hair and soap scum narrow the waste of a bath, shower tray or basin long before it blocks. Clear the grid and trap first; if several fittings are slow, the blockage is in the drain.
Caution
Have a full or overflowing tank emptied by a registered contractor and check the drainage field. In England, GOV.UK says to remove sludge before it reaches capacity, at least once a year or as the maker says, and that the sewage must not cause pollution.
Where you smell it tells you where to look. Near the tank or drainage field points to the tank or field; inside the house points to a water trap or the soil vent pipe.
Describe the job and get a fixed price from a local drainage engineer.