Ever stood in front of the whiteboard with a group of your co-workers designing a system, but found yourself in that awkward position where none of you were able to answer whether something would be fast enough? In this talk, you'll learn how to combine base rates to answer challenging questions in a jiffy on pull requests, technical reviews, or in meetings, for example: Can we in the 5-second critical window on a regional failover, do a snapshot-and-restore of Memcached to the other region? How much overhead should we expect a proxy to incur? How far is this system from the optimum performance? With the methodology from this talk, you'll learn to quickly estimate expected system performance instead of building them first!