The URL causing the highest latency for users is /slow1.php on the server with IP address 172.16.20.3. This is evident from the first exhibit, where /slow1.php has the highest average server latency of 502.12 ms compared to all other URLs. In the second exhibit, the server 172.16.20.3 has a significantly higher average server latency (158.36 ms) compared to 172.16.20.1 and 172.16.20.2, corroborating that the /slow1.php URL on 172.16.20.3 is causing the highest latency.