Your WiFi and upstream are fairly well matched. When you say 10mbit, is that using the site that's causing problems or is that a speed test? As a speed test it

Your WiFi and upstream are fairly well matched. When you say 10mbit, is that using the site that's causing problems or is that a speed test? As a speed test it seems slow, unless there's some congestion on the WiFi or uplink. If it's the speed you're seeing the the site causing the problem, it makes me suspect that there is a issues with selective ack that could cause the issues you describe. Maybe some jitter or out of order arrival. Probably need to see some packet dumps to see how it's behaving and compare the ones that work well and the one that doesn't.