There are only a handful of slot restricted airports in the US. JFK, LaGuardia and Reagan Washington National are level 3 airports and Chicago O'Hare, LAX, Newark and SFO are level 2. Every other airport you just file your flight plan (for an IFR flight) and go.
I think it’s just all marketing. If there’s a 7:38 departure between A and B and a 7:54 departure on a different airline you’ll pick whichever one fits your schedule better if you’re not locked into a loyalty program.
Airlines have immense data files on schedules. They know the average taxi times for each flight and build that into the schedule. If they start the day at 6am with a 2 hour flight that averages 10 minutes taxi time at the departure airport and 8 minutes at the arrival airport and plan a 50 minute turn then the next flight leaves at 9:08.