Just brainstorming a bit...
From what I gather, there seems to be a feeling that Ferrari can pull out vastly more performance on a lap per lap basis, but only for a very limited amount of time.
AMuS did an analysis of Spa in 2017 and 2018 where they conclude that even if you factor in that 2017 Mercedes was probably running less wing and in 2018 more, there was something magical about how that Ferrari continued to pull/accelerate even after he got out of the tow. The end speed was similar, but the Ferrari got there much quicker. Some also report that there is a screeching sound that can be heard on very few instances, possibly when Ferrari deploys what ever they have at full tilt.
- Mercedes feels that this advantage can not be attributed to aero, as if that were the case, the benefit would be felt on a more consistent basis and not a momentary boost. It perhaps also wouldn't explain that weird sound.
- there is only so much that you can store and use (legally) from the battery system. Assuming both Mercedes and Ferrari have a fairly good idea on where and how to deploy that energy at its best, I can't see how it could be in that area either. Not by the amount of performance boost Ferrari can get out.
- fuel... well, if it's within the fuel, they should be able to deploy it more often and not for the same duration at all times. At least assuming they have just the one fuel they are using and not a separate tank with "special" fuel. (which would be illegal anyway).
What about if Ferrari somehow store air pressure that they can deploy? Would there be some benefit if the quantity of pressurized air would be large enough, dense enough and could be somehow deployed to increase boost on some level? I presume what's at odds with this, is that at the end, you are still going to be limited by the amount of fuel flow you can use. Could pressurized air be somehow be used differently? Just throwing this one out there...