This is a pretty good tutorial for what you want.
http://www.hackinsight.org/news,38.html
Also check out WiFiphisher.
Just make sure you are doing this on your own network :) or have permission!
It doesn't seem like the antenna would be the problem(unless your unit is defective) I have no problem picking up clients farther than 15 feet with the pineapples antennas. Maybe try reflashing the firmware and try again. It's hard to say what is causing your issue without more info.
How big is your sd card? do you have anything running when it'S slowing down? Can you post your ssl strip log?
I have several infusions installed and have never had any issues running multiple infusions, I ssh into my pineapple quite often and haven't experienced what you are seeing.
Sometimes under /sys/bus/usb/devices/usbx there is a power setting you can mess with, I'm not seeing that in the pineapple though so I don't know if it's possible.
I'm pretty sure the pineapple will only find networks in the 2.4ghz range, ac is 5ghz. As far as I know there are no supported 5ghz usb dongles for the pineapple, but I could be wrong :)