What app is killing my WiFi download speeds?

About two weeks ago, I broke the camera lens on my Galaxy S5 (Japanese docomo SC-04F). I took it to the docomo shop and got a new phone (S5#2) for only $50, but after some initial setup, I noticed the phone dropped WiFi connections, would not connect when a known WiFi spot was nearby, and the download speeds were only 5% of the still working old S5 and my other devices.

So, docomo gave me a new phone (S5#3).

The third S5 was initially fine, but after loading all my apps, the same thing has happened! And this was no problem on the original phone even though it had even more apps. I'm wondering if one of the chat apps is the culprit (Telegram, docomo mail, Inbox, LINE, messenger, WhatsApp, KakaoTalk, Skype, Viber). Telegram, docomo mail, and Inbox (Google, not yet registered) are the only major new apps, and really only Inbox is totally new.

How can I figure out which app (or setting) is causing the problem?

 

Thanks for the reply, and that is exactly what I've started doing, but after the top ten most suspicious, most recent apps, there is no change. My download speeds are still 30-50kBps compared to 300-800kBps before and with my other devices still.

I've made an adb backup using

and I'm thinking that after another dozen or so app uninstalls that I'll do another 'nother 'nother factory reset and try to find the culprit as I'm building up the system. I'm pretty much at a loss as to what it could be, though. The system is not much different at all than what I had on the original S5 before my friend poked its eye out . . .

BTW, do you, or anyone else, know if the adb backup I've created will restore my widgets and shortcuts on the home screens? Those took a lot of time to do -- again -- again . . .

And as a follow-up, couldn't I start a WiFi speed test or some long download and then go into Application Manager and note all Running Apps and see the most likely candidates? It seems to me that really nothing should be able to cause what is happening to me -- again.

 

One of the things that I had changed on my phones was NFC. I had turned it on on both of the new phones, and I remember when turning it on somewhere it said that it would use no battery, but I had noticed NFC appearing in my battery usage statistics , so I turned it off and checked the download speed, but there was litle difference, and then I pulled my SIM card (NFC won't work on docomo phones if you remove the SIM), and BAM!

My download speed went from 30kB/sec to 1340! That's 40-times as fast! Before this test, it had been bouncing around between 10 and 60. After that, though, it has returned to about 50kB/sec. again . . .

I'm about to test my adb backup, and if that restores a few widgets and shortcuts I've removed, then I'll do a full reset and try these tests from the bottom up.

 

To bring some closure to this, I did a factory reset of my S5. When the phone restarted, I was surprised to see that it downloaded almost all of my apps. It missed only a dozen or so out of 250 or so. Before it had re-downloaded only 20 or so, but that was on a fresh, replacement device, not the factory reset of an existing device. Lesson One learned.

So, I did not need so much to restore the adb backup that I had created. I just took the opportunity and started somewhat fresh, doing WiFi checks often along the way, and I was also less aggressive in removing system apps.

After reinstalling almost everything and setting up almost everything, my WiFi is still doing fine.

I think that removal/turning off of one of the system apps may have been what caused the problem. I am most suspicious that turning off "DOCOMO Initialization" was the culprit, but I don't want to do it now again to check . . . I thought it was an initialization script for first use, but it seems to be an initialization for every boot and/or use of docomo's network.

In any event, all is working fine now. Thanks Pugs1957 for the attempt at helping.