I enjoy using Iron Man Infinity War (IMIW) but have long-suspected that he takes longer to get a bar of power than most other champs, so I wanted to test this suspicion.
To test, I took a series of champs to fight RoL Winter Soldier. I recorded these trials. In order to have a successful trial, my power gain had to come exclusively from hitting the opponent. This means no blocking or parrying. Also, since IMIW has 2 hits on his second Medium attack, I had to stick to MLLLL combos for all champs in the trial. Finally, the champs tested were not allowed to have any power-altering portions in their kits. This means champs like Psylocke, Aarkus, Cyclops (Blue), and Hyperion are ineligible.
To get a baseline, I took a 5* Rank 5 Sentinel and Void and recorded their hits. In both cases, it took exactly 36 hits to get to an L3. This tracks with the understanding that champs of the same rank and rarity have the same inherent power gain rates.
So I moved on to Rank 3 5* champs. I started with Iron Fist:


It took 39 hits to get to an L3 using MLLLL combos and no blocking.
Then Karnak:


It took the same 39 hits to get to an L3 using MLLLL combos and no blocking
Then IMIW:


IMIW took 43 hits to get to an L3 using MLLLL combos and no blocking
I think moved up to r2 6* Champs, starting with Cap IW to get a baseline:


It took 41 hits for an R2 6* to get to an L3 using the same criteria.
Then IMIW:


It took IMIW of the same rank & rarity 46 hits to reach an L3.
So it seems that IMIW gains power more slowly than other champions despite not having any mention of such an aspect in his kit. Without knowing exactly how much power was gained on any hit, it's impossible to nail down the exact shortage IMIW is seeing, but looking at the data collected,
it appears that IMIW gains roughly 10% less power than other champs. Is there a reason for this? Is there something I'm missing? I'm interested in hearing what others have to say. Thanks.