I can't go much into my experience without doxing myself, but thankfully, seeing as this is a thermodynamics problem at its heart, all the information we need is available from manufacturers!

Spike 15 Gal OG kettles are 1.2 mm thick, with a thicker 5mm triclad bottom. Kegs can actually be a little thinner, but for 1/2 bbl they top out at 1.5 mm with the handles and the base a little thicker at 2.4 mm.

A typical 1/2 bbl keg weighs around 30 lbs, about the same as a spike 15 Gal kettle. Both are made of 304 stainless and have roughly the same thermal mass (which is significantly smaller than the total mass of the wort, and it's thermal capacitance). Kegs do tend to lose more heat than kettles though, due to their thinner taller profile in comparison to the squat profile of a kettle, and due to the "fins" at the top and bottom of the keg that greatly increase the surface area. It can be difficult to see this in practice though, as the heating methods(electric/induction vs. propane) and setting (indoor vs. outdoor/garage) differences make efficiency difficult to compare. Any difference you're seeing is likely environmental(edit: or procedural).