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?