Internet Slowing down sometimes when other devices connect

Sometimes when i'm playing a game like league of legends, other people visit and they automatically connect to the internet with their phones since they have been here before and I gave them the Wi-Fi password. When that happens, my connection to the game completely drops, unable to play since its a online multiplayer game and it just attempts to reconnect. I either turn off my Iphone 6s Wi-Fi off to have the internet working about but sometimes I have to turn multiple devices Wi-Fi off. All the phones are being unused and it still affects the Wi-Fi. My internet specs are 25Mbps down and 2Mbps. Is there anyways to fix this?

Use current phone SIM card to activate old iCloud locked phone?

I found my old iPhone 6 and I have a new iPhone 6. I don't know where the Sim card is for my old one. Can I use the same card from my current iPhone 6 and put it in my old iPhone 6 to activate it and then take it out? After I activate it will I be able to take out the Sim card and put it back the my current phone and have two WORKING phones (one with out and with SIM card) without it deactivating or messing up my current phone?

IPhone charger works fine in the wall, extremely touchy in a computer. Why?

My iPhone charger works flawlessly when plugged into the wall. It's a USB charger that connects to a wall piece. When I try to charge my phone through my laptop (or desktop)'s USB outlet, it gets extremely touchy. Even the slightest movement will cause it to switch rapidly a few times between being plugged in and not. This is not just on my own personal computers, but any computer I have ever tried (roughly 10 different ones?).

Could some explain why?