@jinlsui 

Hi, for the 800m/s listed by Dell XPS9570,  I think you mean the link rates.
if yes,  that is the speed at which clients and nodes communicate back and forth with each other.  Typically internet speeds on 5GHz are about half the link rate.  Though with advancements in technology that is not always the case any more. 
 
For the X5700, the link rates could reach 3843 Mbps (802.11ax, HE160) for 5ghz-2/1201 Mbps (802.11ax) for 5ghz-1;
But Now, the link rates on Dell XPS9570 is only near 802.11 ac standard(866m/s) so that I think, somehow the Dell XPS9570 link rates limited your final wireless download speed.
Have you checked whether there is any new driver available for an upgrade?

TP-Link wrote

@jinlsui 

Hi, for the 800m/s listed by Dell XPS9570,  I think you mean the link rates.
if yes,  that is the speed at which clients and nodes communicate back and forth with each other.  Typically internet speeds on 5GHz are about half the link rate.  Though with advancements in technology that is not always the case any more. 
 
For the X5700, the link rates could reach 3843 Mbps (802.11ax, HE160) for 5ghz-2/1201 Mbps (802.11ax) for 5ghz-1;
But Now, the link rates on Dell XPS9570 is only near 802.11 ac standard(866m/s) so that I think, somehow the Dell XPS9570 link rates limited your final wireless download speed.
Have you checked whether there is any new driver available for an upgrade?

@jinlsui Have you connected the Dell XPS9570 to other wireless networks to check the link rates as well the wireless download speed since 450 m/s download speed out of 800m/s wireless link rates is quite normal?