Water pressure will be the same, volumn will be less because of the reduction in size. When run to a single fixture, it is usually enough. Most supplies are 3/8" anyway.
The bigger concern would be sizing the larger runs before you branch to 1/2" If you have more than one bath in the home, 3/4" will be too small.

The chart below works well for copper systems. You may want to consider the smaller size of the CPVC and adjust.
When using pipe thread sealant on plastic, make sure it's compatible. Not all of them are. Terry

• Water pipe sizing chart