The dweebs on here will try and tell you that 5lbs is a small fraction of the total weight so will make little difference when winching up a steady fire-road climb, which is true.
However on singletrack, where the bike is being accelerated up, down, forwards, backwards and side to side many times a minute or even second by you, your brakes or your tyres, 5lbs makes a massive difference.
But of course there are FS and FS. Lighter FS usually have steeper angles which makes for a better pedalling position and a load of other handling factors. It depends on the trails too and the style of corners etc. My Orange 5 (28lbs) is much much faster around Swinley singletrack than my Kona Heihei (21lbs), because the geometry works much better for those trails.
On the other hand, my Patriot was 31lbs new and pretty slow. I made the tyres tubeless which saved maybe 0.5lbs and it’s now much faster and more lively, because the rolling resistance is now significantly lower.
When I bought my first FS I tested one of the first FSR 120s and a Fisher Cake. The FSR was maybe 2lbs heavier, but felt an utter dog on the climbs, whereas the Cake climbed like a spanked goat. They had very different angles.