My Laaggy internet

Hello, like my internet used to really fast. for some reason, it's slow as hell.
im using a 15Mbs cable internet that is extrememly fast. i have 2 computers connected on ethernet and 1 connected to wireless. the thing is, on one of my ethernet connected computers are extremely slow, while the other ethernet connected computer and the wireless computer's internet is pretty fast. ive been going to speedtest.net and other internet speed testing, it's been pretty fast, a 14-15Mbs download and a 3Mbs upload.

When i play my games on Garena, thats when the problems arise. other people ping me and they say that i have less than 200ms. but when i play, it has horrific delay(3-5 secconds of delay). other people say that they dont feel my lag. For sure, something is up with MY internet.....
other tasks like streaming videos and downloading games have been really fast.
can someone PLEASE HELP ME?????????