Yr questions r normal... Every 2 or 3 days, someone ask same.

Yr case, u must 1st determine if it is bad unifi or bad wifi.

And u will not know until u do speed test with Ethernet. Borrow laptop or some device to test wired torrent. If still bad, u hv case to complain to unifi1. If good, u know u need to deal with wifi.

I'm no expert but from the hundreds of similar cases I read in this section, I will think u need either a super Wi-Fi router or install wired lan. Big house, no shortcuts. It's a tech issue.

Listen to bro AVFAN advice. I will first say that you need a stronger more expensive router that have better transmission power, stop using the TP Link WA850RE, and get a better WiFi receiver.

Tried a different client? Ever since uTorrent messed up by bundling crap with the free versions.. closed source, ads and hidden crypto miners.. I've since switched to qBitorrent. It's open source and I use this to download..cough..cough.. Linux distros.. ehem... No unusual speed drops and highly customisable provided one knows what they are doing.. 

Multiple attack points.. Emails, calls, live chats, visited TM Point over half a dozen times..I've got more than 10 report numbers and made 2 long handwritten reports in person.... Till they agreed. Even then, the TM Point staff still managed to mess up the switch.. Downgraded my already free Jumbo pack to Jumbo Lite and then charged me a RM10 downgrade fee for their mistake. Time's money so I decided to call it a win and now looking at the best way to get TIME to move in.. There's a condo beside mine and it's already TIME equipped. So shouldn't be too hard..
And you missed my point. I'm aware of the client you use. My point is that since all the slowdowns happen when said client's in use, but most others using a different client do NOT experience the same. Once again.. Have you tried another client? Preferably the one I suggested as I use it and can vouch for no slowness as you experience. In fact, it adapts it's speeds based on parameters and network usage..

Ditch the range extender and weak receiver.
For starters, you can try getting a better wifi adapter like this:
https://www.lazada.com.my/tp-link-ac1300-du....7f1c206fLys9VI

If the connection with this adapter is not good enough, immediately utilize the 14 days easy return on lazada and we can try something else


the other 2 users in your home.. do they connect directly (wifi) to the ASUS AC1200G?