While in the US we often say "customs" to mean "customs and immigration," note that these are different things.
Customs in Europe (control of goods) is just a matter of walking through the Green Channel - Nothing to Declare line. Unless you're "randomly selected for addition screening" (possible but rare), you're done.
Immigration in Europe (passport control) is not "daunting" but can take time. How long it will take depends on how many officers are on duty and how many other flights are arriving. The only thing to make sure of is that you use the lines for non-EU passport holders. You just line up with everyone else and wait your turn. You then pick up your checked luggage, go through the Green Channel, and you're done.
You will have go through these steps on arrival whether you stay at an airport hotel or in Haarlem.
For that 7:30 AM flight departure, you'll want to be at the airport at 5:30 or 6 AM (check with your airline for recommended times). Staying in Haarlem would mean far too early a wake up call for me, but you may feel differently. I'd drop my bags at the airport hotel, then go to Haarlem by bus or train, then return to the airport hotel to sleep; this will give you about an hour of extra sleep vs staying in Haarlem.