What could be causing low pressure in my new faucet, a Moen 87690SRS?
Sometimes replacing fixtures will break loose crud that will end up in 1 of 3 usual places:

At the shut off valve/connection hose

Inside the cartridge

At the aerator screen
Start at the aerator, its the easiest to check.  Make a water check with the aerator off.  You can flush out the aerator in reverse at that time.  Be careful to reassemble in the reverse order, you wouldn't want to leave the restrictor plate off accidentally.