They locked the old thread for this discussion. I am starting a new sthread to continue. 
old post: https://community.tp-link.com/en/business/forum/topic/608700

I have exactly the same issue. Eventhought speetest is good, browsing often becomes slow/laggy when ER605 is used. The issue goes away when I bypass ER605. This is the case for multiple device in the house.
My ER605 is used for failover with two WAN ISP. 

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ER605 slow/sluggish/laging browsing

2023-05-10 16:13:23 - last edited 2023-05-11 01:36:11

Model: ER605 (TL-R605)  

Hardware Version: V2

Firmware Version: ER605(UN)_V2_2.1.2 Build 20230210

Hi,

Looking for help. I have ER605 router for some time, directly plugged into ISP modem (dynamic IP configuration), which then redistributes connection through cat 5e/6 cables around the house.

I have been experiencing good download/upload speeds - as dedicated from ISP 500/50 mbs, however whenever I use any browser to open a website (either directly plugged via Ethernet cable - all cat 5e/cat6 less than 20m in length, or via wifi - TP LINK deco M4), like 80% of the time, the websites take some time (like 10-20 seconds) before they start to load (like the router is trying to check something before it actually starts to establish a connection with the website server).

I've tried restoring to factory resets, updated to latest firmware, but nothing helps. I went back now to old totolink router which works fine (no sluggish/lagging response), except it does not manage 500 mbs bandwidth..... So it's not the cables/wifi/or any other hardware. Looks as ER605 seems to be the problem in the whole system.

I also tried:

- turning off/on all firewall protection

- vlan on/off

- on/off bandwith control/optimization function

- and basically all other router functions which seemed that could affect the router's bandwidth/connection establishing functions.

In some other thread (can't find it again now to share link sorry) I've seen someone suggesting that the router may use slow echo server - not sure how to change the default server - could use guidance on how to do/try it.

Overall it looks as the router is trying to do something before it establishes a stable connection with a website server (or any other actually, this sluggish response shows up also during use of other apps: i.e. game servers or others). Once the connection is established it works fine with good speeds for a while - hence why I guess I have good download/upload speeds.

Thanks!