Can i use an ethernet splitter with an ethernet cable connected to my router?

i have an ethernet cable running from my router to my room. i want to connect to devices with the same cable. that is why i need a splitter. would a splitter work if i wanted to take the existing cable in my room and attach the splitter then use the splitter to connect my smartTV and my xbox 360? also would the splitter slow down the internet on either devices?