Previous studies have shown the WiFi, as the dominant last hop access to Internet, has become the weakest link in the round-trip network delay. Therefore it is critical to understand and minimize the WiFi interference in order to reduce the WiFi hop delay. For the first time in the literature, this paper defines an intuitive and accurate metric to quantify the impact of interference on each actual packet. For each packet traveling through the access point, it measures the percentage of MAC layer delay wasted due to neighbor APs' interference. This metric is defined based on a packet's various (measured or inferred) timestamps and can be measured with a small kernel modification on a commodity AP with little overhead. Our 29-AP two- month measurement results in the wild show that this metric is a strong indicator of interference's impact on WiFi hop delay. Using this metric as input, distributed channel selection on individual APs reduces the median WiFi hop delay by up to 5X. Collaborative optimization on multiple APs reduces the overall WiFi hop delay by 5X compared to the default channel.