A gas spring-powered fastener driver includes a first chamber, and a movable piston positioned within the first chamber. The fastener driver also includes a driver blade attached to the piston and movable therewith between a ready position and a driven position. The fastener driver further includes a second chamber containing pressurized gas. The second chamber is in fluid communication with the first chamber via a flow passage. The fastener driver also includes a throttle mechanism configured to throttle flow of the pressurized gas through the flow passage.