Incoming water pressure is at 80 pounds. Infrastructure has had locked air problems in the past; but seems to be OK now. All tubs, sinks, etc. has a very low flow rate. Measured at 1 gallon every 30 seconds.
In the morning, the flow rate is much higher - about 5 gal per minute but if you fill one tub - it drops to 2 gpm. Running water in more than 1 sink tub etc will reduce flow to less than 1/4 gallon per minute.
I've run water in upper story of house to try to free air lock. No noticeable amount of air released.
The water filter has 80 pounds of pressure at measuring point. Filter has been replaced. House does not have a pressure reducing valve (but should).
What to do?

It's not air lock causing your problem. You have a volume problem. What type of pipes do you have and how old are they? Volume and pressure. A system must have both to operate correctly. The problem with rusty pipes isn't a pressure problem, it's a volume one. Let's say you have 50 PSI at the water meter and a 3/4" galvanized service that has built up rust that chokes it down to 3/8".
You will still have 50 PSI when you open a faucet, but, flush a toilet or open another faucet that calls for more volume to feed the extra demand and the pressure fails.
Cheers, Tom

It's not air lock causing your problem. you have a volume problem. What type of pipes do you have and how old are they? Volume and pressure. A system must have both to operate correctly. The problem with rusty pipes isn't a pressure problem, it's a volume one. Let's say you have 50 PSI at the water meter and a 3/4" galvanized service that has built up rust that chokes it down to 3/8".
You will still have 50 PSI when you open a faucet, but, flush a toilet or open another faucet that calls for more volume to feed the extra demand and the pressure fails.
Cheers, Tom

Pipes are copper and plastic; so I suspect that a partially closed gate valve may be at fault.