MACsec protects automotive Ethernet links, but a secure channel is only usable once the MACsec Key Agreement (MKA) has finished. We evaluate how quickly MKA with automotive optimizations (pre-selected key server, immediate responses, hello-time ramp-up) can complete. Starting from measurements on a Raspberry Pi demonstrator, we simulate the influence of start-up offsets, processing times and cyclic message handling as found in AUTOSAR stacks, and compare the results with an experimental setup of two NXP automotive MCUs running an MKA implementation following the TC17 profile. Key findings: the start-up offset between nodes significantly influences the time to key agreement, the fastest agreement counterintuitively occurs when the key server starts late, and cycle times above 5 ms are not reliably fast enough.