We will be arriving at Fort Lauderdale Airport on a Monday and will rent a car and drive to our hotel at Delray Beach. We'll probably leave the airport at around 4:30pm. What will the traffic be like at that time in that direction? Would it be better to go 1A or 95?

I'm no expert, but I have much worse luck in rush hour on I-95 than the other routes. I don't think I ever hit it when there has not been a major wreck and traffic is at a total stand still. You've also left off US 1 (not the same as 1-A, which honestly may be less slow than either of the other two.

But let's see what someone more local says.

Also when are you going? Now till Easter is the worst for traffic.

I hate driving I-95 especially @ rush hour so I avoid it. When I leave FLL airport car rental parking lot( I use National Emerald Aisle for convenience of pickup/dropoff) I get on US1( federal highway).Go North on US 1/Federal Highway. Exit @ 17th St.Causeway or Sunrise Blvd or somewhere along there. This takes you to A1A(or called North Atlantic Blvd in some places). Then just turn left(north)on A1A/Atlantic Blvd & drive North until you get to Delray. It will probably take you about an hour. Lots of stop lights in some places. It's very pretty driving along the ocean.
Where are you staying in Delray Beach?

Another thing you cld do is go directly from the airport car rental parking lot to Charlie's Crab on the intracoastal in FLL, have a nice early dinner out on the patio & then head north to Delray Beach (same route). We often do this.

A year ago there was nasty construction going on in the northern part of that route of I95 - it had the look of construction that would continue forever, but hopefully some local can give you an update. Because of this, rush hour was worse than usual when we were there last Feb.

95 may be slow for the first stretch north of FLL,then pick up speed. 1 or 1A or A1A are all slow, slow, slow, with many traffic lights. Suggest 95.