How to define a slow drain.

How do I decide if I have a slow drain in my shower? Is there a standard gallons per minute drain or some other method to see if it is ok? I just put in a new tun and I am not sure if the drain screen is making things slower, if the old tub drained at that speed, or if I am just plain nuts (which my brother suggested when I told him I thought the drain was slow). Any help would be appreciated.

A shower drain should easily keep up with a single showerhead. Now, if you have multiple heads, at some point (if this is a tub/shower with a 1.5" drain) you'll exceed the capacity. I doubt you'd do that in a residential shower with a 2" pipe in a standard shower.

If I had to guess, I'd say my tub drain runs at about 12 gallons/minute (it's about a 40-gallon tub when filled and drains in about 3-minutes) into a 1.5" drain.

Drain screens may slow down a tub drain.
As already stated if it won't keep up with the shower something needs to be done.

I know I need to clean my shower drain if I ever find myself standing in a puddle of water when my shower is on.