Is there a point when the fixture drainage is so slow its considered unsanitary? I don't see any mention in any plumbing codes?

Your tub should drain in a matter of only a few minutes, 5 at the most! Most common slow drain problems in a tub come from hair and soap caught in the drain. Most drains will need a cleanout from time to time using a plumber's snake if there is a lot of hair getting caught. Other problems could be a full septic tank or a broken or collapsed sewerage line out of the building. You don't mention what you have other than a slow drain.

Is this a tub only , or a tub shower? A good test to see if your tub is draining acceptably(if you have a shower head) is this,, if the tub drains all of the water from the shower head, the tub is drianing okay, but if the tub backs up when only shower head is running, sounds like you have a blockage in the drain. Many tub spouts will put out more water than the drain can take at one time. Codes state drainage fixture units, but they are only for sizing of drains, they don't aproach the subject of clogged drains, hence nothing to be found in the codebooks.
As far as unsanitary, as long as it drains somewhat, its not stagnant water, no different than taking a bath.

There are other fixtures but 16 tubs are being installed in an 80 year old building. It's a remodel but a lot of existing underground is to remain (which has been snaked several times & camered once). All the rough plumbing passed (including the air test - of course). Now the inspector, since he is aware of all the snaking of the existing, is filling the tubs half way and watching them drain one by one. More than half the drains stopped draining completely. Some are draining so slow its making the inspector totally uncomfortable. (One tub drained then filled back up with brown water right in front of him). The plumbing work hired to do is complete and all the rough passed but the existing must be really bad. Need "inspector pass" idea? -between a rock and a hrd place! Anyone been here before?

Hate to say it, but it looks like your in for some seriouse repiping or snaking costs. This may hurt. No magic fix here.

Yeah, that's the reality. I may have talk with the owner of the building and explain. At least I told him before I started that remodeling a really old building with an old infrastructure is opening a can of worms and more work may be needed. Thank you for you help/opinions.

The inspector night have passed the remodel but I bet by this time he balked like hell about signing off on the C.O.
Dad nailed! My take on it?

Now the inspector, since he is aware of all the snaking of the existing, is filling the tubs half way and watching them drain one by one. More than half the drains stopped draining completely. Some are draining so slow its making the inspector totally uncomfortable. (One tub drained then filled back up with brown water right in front of him).
80 years old huh? (Almost as old as me) It's my bet you'll be tearing out those 16 tubs you just installed and ripping up the floors to replace those old cast iron drainlines. But wait, there's more!
the inspector, since he is aware of all the snaking of the existing, The inspector and your plumbers might be too uoung to remember how a drum trap's built,(see image) That's right, sports fans, they can't be snaked past the trap if you're attempting to snake from the tub.
And you got drum traps in a 80 year old building. I can see the inspector making you change the drum traps for "P" traps and replacing the old rotten drainage all the back to the stack vent.
As Dad sez, "This may hurt!!" Good luck, Tom