Feelings aren’t really a great gauge. Otherwise we‘d still be on 17, 19mm tyres and 120psi on the road. If you’re only climbing the smoothest gravel roads then yes, the hardtail will probably be more efficient and thus faster. As soon as it gets bumpy the full sus will probably be more efficient and faster.
Pinkbike did a ‘fieldtest’ some time ago where they tested XC bikes from Cannondale, Canyon, Specialized, Trek, and two DC bikes, the Transition Spur and Yeti SB115. In this mix was a Trek Procaliber in different categories and discussed the overall ‘best’ XC/DC bike. They did a short smooth climb and a loop on a XC course. On the smooth climb the Trek Procaliber (Hardtail) placed midpack among the Epic Evo and Supercaliber (2.43). The Canyon Lux was fastest (2.32) with the Epic (2.38) behind. I can’t see how the Lux could be 10 seconds faster over such a short climb, but anyway - bro science yo.
They did another lap on a XC trail with a steep and bumpy climb. Of the top of my head I remember the conclusion being that the more modern geometry was fastest, but I could be wrong. As far as I remember they didn’t include the Hardtail on this lap (why would you?). This ‘field test’ is of course for entertainment and not really a scientific comparison. If you want to have a look, here’s the playlist: 9 XC Bikes (& The Grim Donut) Hucked To Flat | 2020 Field Test XC/DC - YouTube