Ok a little follow-up on this matter. The integrated rpi3 wifi is a mess. But I think I am starting to get it figured out.
1st step , I set the region for the wifi using this link : Raspberry Pi 3 with OSMC unable to use wifi channels 11,12,13 in UK - #6 by lyonacre
This improved max speed. And allowed me to try less crowded channels.
2nd step, I used sudo iwconfig wlan0 power off and it made it more stable. Using this thread : Pi 3 WiFi unstable - #7 by sam_nazarko
Lastly, I have changed advancedsettings.xml as advised on Kodi Wiki for 1GB of ram devices. Here: http://kodi.wiki/view/HOW-TO%3AModify_the_video_cache
As of now, everything looks finally smooth. But I will report on that after a few days of testing.