When more than one drain is affected

Several fixtures are draining slowly or behaving unusually at the same time.

What this page cannot establish

noticing this establishes that you noticed it — it does not establish a cause
this page cannot establish the location of an obstruction
this page cannot establish that a shared run is involved rather than several separate issues
this page cannot establish the condition of the buried line

What a professional will be trying to work out

These are the distinctions a professional makes when they attend. They are questions, not conclusions — and knowing them lets you follow the conversation rather than just receive its result.

whether the affected fixtures share a run downstream
whether the lowest fixtures in the building are involved
whether one fixture backs up when another is used
whether the pattern is consistent or changes day to day

What is worth writing down

These details are cheap to record now and impossible to reconstruct later.

a complete fixture list marked affected or unaffected
which floor or level each fixture is on
whether one fixture responds when another is used

Evidence that would change the next decision

If you are deciding whether something is worth paying for, these are the things that actually move the decision.

a written list of which fixtures are and are not affected
whether using one fixture visibly affects another
the level in the building at which affected fixtures sit

What would change the answer

a written list of which fixtures are and are not affected
whether using one fixture visibly affects another
the level in the building at which affected fixtures sit

What to do next

Understanding why fixtures affect each other is what makes the pattern you are seeing interpretable.

Understand how drainage and venting connect fixtures

If your situation is different

Only one fixture is affected: When one fixture is affected
It has reached the point of backing up: Active backup

Gurgling
Gurgling and multiple slow drains are usually the same observation.
Contractor visit
The fixture-by-fixture list is what makes a visit efficient.