Please suggest me, How can I limit internet speed of certain devices connected to my wifi?
Actually this depends on the type of router you are using. Most of the routers have bandwidth control function but, they will allow to control the speed of all connected devices at once.
These restrictions only work if your router supports it. In the US, your ISP usually doesn't provide you with a router/modem/switch or they give you a bad one.
In the UK the ISPs provide you with a combo device of all three however the advanced functionality is usually restricted unless you buy a 3rd party router/modem/switch