How do you measure performance of the Legal Department? Which KPIs do you use and which data do you collect?

It likely depends on which aspect of the Legal Department work you are looking to measure.  For litigation, that may be the cost of settlements/judgments; for IP, perhaps the cost per patent/trademark filing & cost over the life of the registration setoff against any licensing revenue for same, for employment, similarly metrics on settlements/judgments, etc.

For commercial staff, metrics I've found useful in the past are of course the total number of requests, time to allocation + time to closure, and then broken down into the functional areas that are important for the particular business (align with Sales/Marketing verticals) such as: Channel, Supplier, Customer, Other (M&A, strategic partnerships, etc), as well as broken down into the type of request/agreement for each such vertical.

For instance, once you know how many overall NDAs you're reviewing and how much time they take - that's like an opportunity for automation and/or standardization (I see OneNDA which is a burgeoning industry standard form has recently released an automated option which could be an easy place to start.  Then measure the number of NDAs worked on by Legal versus the increasing number handled by automation to show that performance improvement in your quarterly decks.

Many other opportunities to show progress and efficiency improvement come once you have those basic measurements in place.