Wireless LANs (WLANs) carry a diversemix of traffic, ranging from delay-sensitive real-time applications to bulk transfers. Using existing QoS mechanisms in high speed WLANs (e.g., 802.11n/ac) presents a tradeoff between maximizing the performance of real-time applications and achieving high throughput. We propose SlickFi, a service differentiation scheme for high-speed WLANs that addresses this tradeoff by leveraging existing dual radio WiFi access points (APs). SlickFi opportunistically adapts channel width on a per-frame basis based on application demands and communicates traffic information across radios to control aggregate frame sizes for improving channel efficiency. SlickFi can be readily deployed on commodity devices using only driver-level changes at the AP-side. We implemented SlickFi in the ath9k driver and using real testbed experiments, show that it can improve aggregate throughput by up to 1.8x and 2.2x over 802.11n and 802.11e, respectively and the PSNR of 1080p videos by up to 3.4x and 1.7x over 802.11n and 802.11e, respectively.