One of the key challenges in future Ethernet-based automotive and industrial networks is the low-latency transport of time-critical data. To date, Ethernet frames are sent non-preemptively. This introduces a major source of delay, as, in the worst-case, a latency-critical frame might be blocked by a frame of lower priority, which started transmission just before the latency-critical frame. The upcoming IEEE 802.3br standard will introduce Ethernet frame preemption to address this problem. While high-priority traffic benefits from preemption, lower-priority (yet still latency-sensitive) traffic experiences a certain overhead, impacting its timing behavior. In this paper, we present a formal timing analysis for Ethernet to derive worst-case latency bounds under preemption. We use a realistic automotive Ethernet setup to analyze the worst-case performance of standard Ethernet and Ethernet TSN under preemption and also compare our results to non-preemptive implementations of these standards.