@Xijinping I am experiencing the exact same thing! i have 400 MBPS down with spectrum coming from a coax line. When hard wired to the Deco S4 on 1.5.0 i am getting speeds of 50MBPS Down which is Reduced speeds by 8 TIMES! Meaning my deco system is basically trash to me right now is this is hardly enough bandwith to run my gaming stream! TPLink has to come up with a hot patch now! 

TP-Link wrote

@Xijinping 

Hi, I see you did fix the speed issue by using a different Deco as the main Deco on the other posts, right?

After switching the main Deco, now the speed issue is back again?

May I know who is your internet service provider and was your internet connection type PPPoE?

Was the speed tested directly from main Deco S4? By 5ghz or Ethernet connection?

@TP-Link I'll answer this one myself too. I currently operate with spectrum. With a connection type of DHCP Connected via Coax. The Decos are being used as a mesh bridge to extend connection. Speed test came from Speedtest.net and was verified between the two connections. IE i tested while connected to the 5ghz on wireless and then i connected hardwired to the Deco. The Test while wireless was 295MBPS Down. The test while wired to the deco after the update was 52MBPS. Currently the deco is running 1.5.0 and i did a speed test before to verify a before and after to note any improvements/Deterioration. The test wired before the update was 193MBPS so still less than the original wireless speed but better than the speed of 1.5.0  i have attached my logs for some form of comaparison as the update is performed within the few hours in 9/30/2021