Looking at a small piece of property that has a brick building, its concrete floor. Building is approx 10’ x 10’ maybe lil bigger.

It has drain in center of building, has sink drain, shower drain and toilet. ( 4 total ) all under slab

When flush toilet it’s extremely slow to go down, doesnt seem clogged, but did hit it with plunger and can hear water throughout other drains.

Its all on septic system in unknown condition.

There are zero vents for the plumbing, ive read where this is important for toilet to flush good ( properly)

Could lack of vent tube cause slow drain ?

If so, how would i remedy this?

If it's never had a vent, what's changed? Find all the clean outs and start snaking.

I want it to function properly, this property got put in during the 60s and was used as a “camp”.

System needs a vent to work properly.

Yeah vents are important. Think of a straw in a liquid and put your thumb over the opening and take the straw out, it holds the liquid. Then remove your thumb and the liquid flows. Same thing with plumbing. It needs air flow to allow proper water flow.

Might have a clogged main line also. Could snake it out with a machine.

Venting is important, mainly to prevent traps from being siphoned dry (think of your toilet).  It could be a vent issue, but I'd put my money into a plugged drain or full tank.

This. I'll add; you could put a vent in the main Just outside the building (maybe off the clean out if it has one)  without tearing up the slab. It's not textbook plumbing, but would off some relief.

In spades.
With no vents the water trying to leave the bowl is blocked
by the air in the drain line that cannot easily escape.

You may be able to 'get away' with a vent that is smaller than the drain line
but around the same size makes everything work smoothly.

The inlet pipe to the septic tank should be a full pipe diameter higher than the discharge pipe leaving the tank, so pushing air out of the way is not an issue.

I assume you mean the vent pipe through the roof should be full sized, I've never see that as code, WI requires a single 3" vent regardless of the number of DFU's.  

Minnesota however doesn't, you can have a bunch of 1 1/4 in vents, as long as the cross sectional area of all the vents equal that of the size of the building sewer. (but toilets have to be 2")

I think it is IPC that is 1/2 the size of the drain.

I have seen many older homes with not a single vent anywhere to be found, that drained fine.

It it is simply a way for the air in the line TO the tank to get out of the way of the rushing water.
Many places allow reduced size vents  even on toilets, but not ALL.
Most require at least one larger vent for each system.