[操作疑難] Cisco 換左DDWRT 之後wifi好慢

用緊Cisco E1200,之前發覺原廠個firmware 自動保留20%網速,仲有個setting太少野唔適合自己用,所以轉左去DDwrt。換左之後Lan線download可以用到原本ISP個full speed,不過發現個wifi 超級慢,原廠個firmware測速 ofta download大概有40 Mbps, 但DDwrt個速度頂多都係得5-6 Mbps,我唔係太識玩路由,無論原廠firmware同DDwrt都係基本用Auto之類既傻瓜setting,有冇Ching 知道點解?