First of all, congrats on getting there! That's an achievement in and of itself.
The primary thing I would focus on is working on the feedback you received after the first round. This should give you some guidance on what are the things that you did well (and ideally you emphasize them more in the second round as spikes) and where you could do better (and you should focus on clearly improving these).
That aside, anecdotically, the tendency for senior interviewers is to focus more on communication and presence. They want to get a sense of whether they would see you in front of one of their clients in a few weeks from now and whether you show an openness to being coached. They might care less about how quickly you work with numbers, which arguably has already been validated by their more junior colleagues.
Best of luck with the prep and reach out if you have any questions!
Cristian
