mainly the kids iphones.......also I have 2 chromecast audio's.  I witnessed the kids iphones being sluggish as hell.  I also have a chromecast audio in my kitchen and outside for my deck speakers.......and I couldn't even connect to them last night.  Very frustrating.  It's weird though as my son can play xbox with no issues on the 5ghz channel.  I even put it back to AP mode and it still would not work well.   I got fed up and attached my Open-Mesh units back up and everything is working like a charm now.  
We do have 7 in our household.....so 7 phones,  6 laptops, an xbox, 3 firesticks, 2 roku's, 1 chromecast, 2 chromecast audios, samsung smartthings, security camera's (both wired and wireless), google home, google home mini (x2), and a few other things that I just can't remember at the moment.  Add the wired NAS, IP home phone system, and Plex pc.