Issue with water volume to master bath

[sbframp]

Okay, so I'm looking for ideas. I have extremely weak water pressure to my master bath faucet. When I use the diverter for the shower, however, I have great pressure out of the shower head. Also worth noting is the fact that I lose psi throughout the entire house when any faucet goes from cold to hot. The pressure gets worse the higher I go in my house which I understand is normal, but I wouldn't think to this extent.

THUS FAR I HAVE CHANGED/TRIED:
-Replacing MOEN 1222 Posi temp cartridge.
-Replacing tub spout.
-Replaced the filter on the whole house filtration system.
-Drained the water heater
-Replaced T&P valve on water heater.
-Checked/cleared the water supply tube connected to the dielectric nipple (hot side) of water heater.
-Checked home water pressure (65-70psi)
-I have a pressure reducing valve that seems to be pretty new with no signs of wear.
-checked all valves throughout the entire house to ensure that they're open completely.
-no signs of water damage at all during home inspection.
-placing house filtration on bypass

I just moved into the home about two weeks ago. Previous owner was contacted and he stated that the water pressure was an issue even before he moved there but he didn't mind because he only took showers. If I were to try to fill the bathtub, it would like an hour due to the weak water flow. Any ideas would be greatly appreciated.

 

[havasu]

What size is your incoming water line?

 

[sbframp]

Appears to maybe be 5/8"? Sorry, not normally the handyman type. I'm sort of spitballing at this point. I've had four plumbers all tell me that they don't know that the problem is. I will say that the piping leading into/out of the water heater is 3/4" PVC and copper mix.