In article , c (TOM KAN PA) wrote:
All the drains in my daughter's house run very slow, including the toilets.
They never overflow or not drain, they're just slow.

They're probably full of frogs. :-)

Whether the toilet is flushed empty or with waste makes no difference.
This sounds to me like it could be the vent pipe.
Is this a possibility?

Yes. The drains could also be clogged. If the vents are not clogged, it's
probably time to snake the drains.

How do you determine if it is indeed at fault?

Climb up on the roof and look for something clogging the vent. You might need
a flashlight to look down inside it, but there's a good chance you'll find a
bird nest near the top, within reach.

And how do you clean it out?

If you can reach the clog, just pull it out with your hand. Otherwise, flush
it out with a garden hose.