When assessing the performance and potential of a startup in our portfolio, what are the key metrics that should be focused on?

Great question, and I think the answer can vary depending on the stage at which the company is at. In other words, the metrics for a pre-seed/seed stage company will differ vastly from a more mature, Series C/D company. I think at the early stage, there should likely be an emphasis on product market fit, execution velocity, the quality of the team, etc. At the later stage once revenue has been established, one can examine things like company burn rates, customer acquisition costs, revenue per employee, and conduct a more sophisticated analysis on revenue as it relates to different product streams. I hope this helps! 

There is no such thing as 'general' metrics. There are number of things that affect diligence on startup such as: stage, location, type of business, industry etc. etc. etc.