Do you have checked the wireless connection speed?

It seems that some TP-LINK products enable the WMM function by default and can not disable it. As I know WMM function can guarantee the packets with high- priority messages being transmitted...

Have you set the correct IP Address/ Service Port / Internal Port in Virtual Server? Then you will make sure the status is enabled.

Hey, I saw http://www.tp-link.com/en/faq-1970.html it is said that Routers and gateways working in their default mode (Router Mode) and AP Mode belong to unaffected TP-Link products. So maybe we need...

Just close DHCP services and switch RE450 mode Range Extender to Access Point.