"quite a few" under-performers sounds a bit dire. I think you need to have a plan for how to improve the performance of the team, which you may or may not want to discuss at interview (as the management seem to have already noticed and are dropping hints.
I think you need to start with roles and responsibilities for each team member, then objectives (and the sum of the individual objectives needs to be the team deliverables). Then what you need to be able to achieve them - coaching, training, etc. And performance appraisals, much easier when the roles, responsibilities and objectives are clearly defined).
But if you set objectives that a team member should be able to achieve at their grade/position, but don't - then you need to bring in HR to resolve. That's the ruthless it. Also if you have this structure in place then your delegation will be more visible to your management, which will help you too.
Unfortunately it's all easier said than done. But in a team-leading role (as this sounds like), you need to lead. Only if you set out your expectations at the start are you then able to demonstrate and measure whether your team members are under-performing. If you get the opportunity to push out an under-performer then you also get the opportunity to replace them with someone better.
I've got two jobs at the moment, in neither of them did I have any technical knowledge at all when I started, but I picked it up as I went along. After 2 years I don't think anyone would question my technical knowledge.
But judicious replacement of people has enabled me to build better teams in both jobs. In one I replaced just one key person, in the other about 1/3 of the team have walked due to misplaced loyalties, but they have all been replaced with better and more motivated people without bumping up the total cost.