Modern CPUs can perform billions of calculations in the blink of an eye and have significantly more memory available, it's not really comparable.
Extremely.
Unescorted settlers run back and forth instead of settling
In wars, 60%+ of units are just loafing around, the AI is incapable of using more than 10 units or so at once in vanilla
Doesn't really make an effort to protect its own cities under siege
It makes peace against players while surrounding their capital with units an era ahead
Fails to take 0 health cities with melee units adjacent
Archers and cities frequently fail to fire on units in range and prefer to end the turn
Policy choices are incomprehensible and worse than what random picks would give you
Unnecessary wonders built all the time
Failing to capture freebie civilians
Builds wonders while being invaded
Builds settlers/workers while surrounded by barbs
Takes so long to regroup if it happens to capture a city it loses all momentum
Almost completely ignores aircraft and nukes
Does not use support units properly, they often run around unprotected
Appears to have little concept of the rock-paper-scissors in melee, mounted, anti-cavalary
Runs streams of units into chokes to die
Places units on spots where they can be killed with no effort, including valuable high promotion units
Settles cities it loses in a couple of turns due to loyalty
Does not send governors to cities it slowly loses due to loyalty
Settles a tile away from rivers instead of on top
Frequently does not attempt to kill off barb camps
Barely scouts on land after the first 100 turns
Frequently 'zones out' late game and ends up not doing anything
When low on gold, will build units only to be forced to disband them a few turns after
Doesn't really attempt to stop people from winning
Declares random joint wars without followups
Religious units walk across the map to try to convert random cities
And the list goes on...