Looking at https://www.speedguide.net/port.php?port=9999 this port info page

It says:

• Several Asus router models use a service called infosvr that listens on UDP port 9999 with root privileges and contains unauthenticated command execution vulnerability. See [CVE-2014-9583]
• common.c in infosvr in ASUS WRT firmware 3.0.0.4.376_1071, 3.0.0.376.2524-g0013f52, and other versions, as used in RT-AC66U, RT-N66U, and other routers

And just to add to the fun here, in my router debug activity page I see:

udp        0      0 0.0.0.0:9999            0.0.0.0:*

Which is unrelated to the smart switch and I think it's saying the router is listening to all udp traffic on port 9999. This must be more than a coincidence with some intermittent reports of ASUS rotuers not working very well with kasa smart plugs!

I have installed Home Automation on a raspberry pi that I'm using for some other stuff too. It works perfectly. Reads all the stats straight away, no delay. Turns the switch on and off all day long, instantly. 

@Solla-topee (A heads up I wrote this as I went through the trial and error process. It's interesting to see the progression here. The TLDR is after properly restarting my router it all started working).

I'm observing the length of delay between switching on and off the switch varies from instant (5 seconds) to 60 seconds (or no response at all) when on wifi. It's always instant over mobile data.

Actions I try to test the effectiveness of a change:

1) Use the app to turn the switch on and off
2) View the energy usage tab
3) Click the KP115 physical power button and wait for the response kasa app to update the status

Setting DNS in router from automatically assigned by ISP to 8.8.8.8 (but not restarting router). This had no effect.

Then changed the router DHCP server to send DNS 8.8.8.8 to devices. Verified phone had 8.8.8.8 as wifi DNS (but did not restart router). This had no effect.

Disabling the router firewall has resulted in it working properly with no delay (after restarting router)!

I haven't been diligently restarting the router between setting changes and I'm seeing some irregularities. The firewall is still off, and I just changed the DHCP back to auto for both DHCP and WAN and it's gone slow again. Right now I'm leaning towards it being the DNS rather then the router firewall.

Router firewall on, router DHCP DNS auto, router WAN DNS 8.8.8.8 -> slow.
Router firewall on, router DHCP DNS 8.8.8.8, router WAN DNS auto -> Fast, responsive and normal.

Everything back to defaults and restarted router, wifi and kasa apps. It's still fast and responsive on both devices. As though this whole time it just needed the router to be reset.

I have triple checked all the settings are back to where they started. It really is working smoothly on both the iPhone and iPad. 

I'm sorry this isn't sounding very clear in terms of what the issue might be yet. Between changing router settings and restarting multiples times it sometimes worked smoothly and others it didn't. As I unwound the changes to try to isolate what was causing it, the second last restart it was slow, then the final restart with all settings reverted to normal it was working perfectly. I really don't know what to say. Turn the router off and back on again until it works? :P

Restarted my router a second time and it's still fast and responsive on both devices.

I will observe that when running over mobile data, I can feel a consistent 200ms delay roughly compared with a responsive wifi connection being completely instant. This is in line with my expectations and confirms for me that it's definitely using wifi when on a wifi connection.

My phone is an iPhone 8 running the latest 14.8 iOS.

I don't have any android devices around sorry to test on but am also using an older iPad mini 3 running 12.5.4 experiencing the same issue.

My network topology is the single ASUS RT-AC68U router which is directly connected to the WAN port from the fibre termination box in my house. There are no other networking or mesh devices on my network other than wifi clients like my laptop, phone, ipad, computer, chromecast and KP115. That is to say, the KP115 connects directly to my RT-AC68U router wifi along with my phone and other devices all on the same 2.4GHz wifi band.

So if I had to guess, I believe the router needed to be restarted for it to work over wifi. My concern is that as I went, the slowness may have come and gone between setting chnages and restarts but I am just not completely sure in the end, given it's now working in the original state. I will keep monitoring the slowness and if it happens again and report back.

@Anthony10 Just fyi I have what looks like the same problem connecting to my Kasa EP10  through my ASUS TM-AC1900. It hangs or is unworkably slow if my phone (IPhone 12 mini) is on the same ASUS network as the EP10 (2.5GHz), and fast if it's on a different network (5.0GHzor cellular). Thanks for figuring that out, and please do keep this thread updated if you figure out a way to fix it.

Cheers,

B