Yeah, I'm not sure if people who recommend IF ever tried to exercise while hungry.
My lifting weights go down by at least 10-15% if I haven't eaten anything that day. Tried lifting in the morning before breakfast - didn't go too well either, just put me in a bad mood because I felt too weak.
You have to allow time for adaptation to the different kind of training... One session on an empty stomach from a fasting newb is not the same as someone who is metabolically adept at fasting training.
Lots of people are exercising while fasted. Body builders, power lifters, and athletes across the strength/endurance spectrum. For things like camping or distance jogging, being able to start at 6:00 and go until 16:00 without food is a game changer.
And, no: people who are training fasted aren't training hungry... Their hunger chemicals dissipate after 20-30 min because they're fasted ;)
I do a powerlifting routine and sometimes work out while fasted (16+ hours). Doesn't make a difference to me to be honest but I haven't tried a 24hr or multi day fast. However, my lifts are quite sensitive to sleep patterns and if I'm not well rested my lifts can suffer 5% or so (15% seems pretty large? That's a 45lb difference on a 300lb squat!).
I am not super strict with when I eat, make sure I get enough. But I agree that my lifts suffer more if I haven't given myself enough rest or time between workouts - that's a bigger detriment to lifting.
Workout after IF isn't an issue for me anymore. But I definitely felt some nausea when I first started while doing some of heavier lifts like squats and deadlifts. I think it's just about giving your body time to adapt.
My lifting weights go down by at least 10-15% if I haven't eaten anything that day. Tried lifting in the morning before breakfast - didn't go too well either, just put me in a bad mood because I felt too weak.