To understand an account that isn't yours, at first, I try to not look at "performance" metrics", and focus on scope and funnel:
- scope: what kind of scale (amount spent, events completed), big trend over time (overall age, seasonality, recent trends.. which periods are most relevant?), geographic & OS spread, mobile UA only or also remarketing, web, promoted social...
- funnel: which events are tracked within Facebook (+which are used for AEO?). is value tracked? (and is it reliable, if often doesn't match internal numbers). What are the different step completion rate.
Once there, I do a "maturity" check
- how solid is the naming convention
- which bidding types have been used/iterated on (mainly MAI/AEO/VO, but also bidding type within)
- number of different creatives
And only then do I start looking at performance (CPI, CPA, ROAS). But to seize if I see improvement in the short/mid term, the 2 bits I put quite a lot of attention on are:
- creative performance (IPM or impression to install ratio)
- audience building, size vs budget. (see Shamanth answer)