For a given effort, you will be slower on a fat bike (except for a few very specific applications which most fatbikes will see rarely or never see)
How much slower depends on the specific bike, tyres and terrain. For me, on a Moonlander with 4.8″ tyres, I estimate a 10-25% disadvantage.
Riding fat requires a specific mindset. It will only be a “royal PITA” if you expect to be moving the same speed as your 10kg hardtail with 2.1″ skinny tyres.