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Will try and help. This can be a number of things so is this a new valve and shower head? If not, how long have you had it? What is the model of your shower head and valve?
I will assume you know you have plenty of flow because you used to have decent flow from the shower head or have adequate pressure at other faucets in that area. That's correct...many models, including Delta have a flow restrictor in the valve body. Delta and few others have had issues with these restrictors. When they are new, since they are mandated to be in the valve, just like the shower heads, you would never know the difference. After years of use, they can become clogged or even "dislodged" from their seat and cause some minor water pressure issues. Aside from the technical side of it, some times the handle on a single lever will slowly move forward not allowing the valve to fully open. Sometimes it can be that easy as reseating the handle?
My suggestions would be to check your shower head and make sure there isn't any build up, since the flow restrictor was removed, at the head itself. Check and make sure, using an Allen wrench that your handle is properly installed and hasn't come loose to the point where it is not fully opening your valve when you turn the water on. Next, if those are all good, check your valve to make sure the flow restrictor in the valve is properly seated and not clogged. You should be able to clean it if it is clogged. I have run into issues with showers that have not been used for a long period of time, where build up in the supply lines from non-use will require a more extensive repair. I would think this should only be examined if this particular shower has not been used for more than a year. I hope this has been helpful. I am curious as to what you end up finding!
Sorry about that, but I'm not sure about a filter? As mentioned, they do have restrictors. Some of the older single lever anti scald valves do have a pressure balancer. It is located in the secondary cartridge. So, you have to shut off the water, take out the main cartridge, and then with some needle nose pliers, remove the secondary cartridge and remove the pressure balancer from the control head. You can unscrew the pressure balancer from the diaphragm and look inside to see if it is worn or damaged. At this point, you can replace the pressure balancer, put it back together and check your flow. This is one of those things that, as suggested above, would fall under more in depth assessment of your pipes and valves. It is certainly something you could do yourself, just would have to be prepared in case you run into more issues.