First time poster!

I have a Moen single handle tub/shower similar to this model: http://www.moen.com/shared/pdf/t2393pt.pdf

I don't have an exact model # (I can take pictures if needed). Single story house and all other hot water output is acceptable. This tub takes about 10 minutes to fill up with warm water. The cold is _okay_ but not great either. I have a large jacuzzi brand bathtub in the master that fills up an order of magnitude faster than this standard tub.

The lever does not need to be pushed in or pulled out.. just rotated for water flow. As soon as the pear shaped knob is pointed 10-15 degrees just right from straight up, the water output from the spout turns to 25% of acceptable. Pulling the pin on the spout to activate the shower head takes a while for water to come out and is also weak pressure.

Bathroom plumbing is on an interior wall with access if needed by cutting drywall from other rooms.

Is this fixable by a DIY'er? Or do I need to hire a plumber?

When you first turn on the water you are turning on the cold water only. As you rotate the handle you begin to reduce the cold water input and increase the hot water. When you reach mid-range you should have warm or equal cold and hot. If the water volume decreases as you turn toward hot then you have a restriction or blockage in the hot water input or you have a failing or defective cartridge. Suggest that you replace cartridge. If that does not work you will have to cut the hot water line. Back flush or clear the line and resolder (assuming you have copper pipe) the line. It is a DIY task if you can sweat solder. Soldering is not particularly difficult. We can step you through if you want to try.

Most Moens use the same cartridge and have different looking escutcheon plates and handles. The cartridge is easy to repair or replace. I think your hot inlet os partially blocked. Shut down the main and open a faucet to blow off pressure. Remove the escutcheon and handle. Moen handles are attached one of 3 ways:1. pop off the center of the clear plastic handle and remove the screw. 2. Use a small Allen wrench to reach a small screw on the underside of the handle. 3. Remove the small plastic plug in the handle and remove screw. Once you are this far you will see a small plate around the stem that has HC stamped on it, above that there is a clip stick out of the valve body, grab with needle nose plyers and pry up and out. Your cartridge should pull straight out. Clean the ports and since there is no leak you can reinstall with a bit of plumbers grease on the inside of the valve. Before reinstalling put a towel over the opening and have someone turn on the main for just a few seconds, this will clear your lines.