Future x-ray observatories will require imaging detectors with fast readout speeds that simultaneously achieve or exceed the other high-performance parameters of x-ray charge-coupled devices used in many missions over the past three decades. Fast readout will reduce the impact of pile-up in missions with large collecting areas while improving the performance in other respects, such as timing resolution. Event-driven readout, in which only pixels with charge from x-ray events are read out, can be used to achieve these faster operating speeds. Speedster-EXD550 detectors are hybrid complementary metal-oxide semiconductor detectors capable of event-driven readout that were developed by Teledyne Imaging Sensors and Penn State University. We present the initial results from measurements of the first of these detectors, demonstrating their capabilities and performance in both full-frame and event-driven readout modes. These include dark current, read noise, gain variation, and energy resolution measurements from the first two engineering-grade devices.