Linksys WRT1900AC v1 slow 5ghz

I recently tried both the OpenWRT and LEDE firmware on my Linksys WRT1900AC v1 but for some reasons the 5ghz wifi is horrible for both firmware. My Roku premiere+ could not even connect to the router if the channel width is set to VH80. I tried various settings (channel and the channel width) but the speed is not even close to the stock firmware. Any suggestion?