I recommend for future needs, split up the devices on to a another wireless AP. Having that many on a home router can and will exceed bandwidth limits technically and can cause problems. WiFi can only handle certain amounts of devices per radio then the bandwidth will become saturated and would probably start to exhibit Lag. You have to realize that even though your connecting 15 devices and have 1 connection between the router and device, in addition to this, there are many other connections going on as well, per each device and the apps they maybe using. One browser can have many connections thru the router to one web site, even though it's one application on the one device. This maybe your common factor in this since you state that you tried a different model D-Link router before. You maybe trying to apply too many devices per one router where you may need some additional wireless APs to help spread out the band width. We seen this before with other users having many client devices connected to only one AP or router. They are only designed to handle so many devices.