How routers and switches affect traffic and speed????

This can become very complicated, but there are two areas that speed, or lack there of comes from. The first, is what everyone talks about, and that is internet speed. This is largely out of your hands, and is dependent on your ISP (Internet service provider). Your ISP will provide some sort of upload and download speed to your modem. Your modem's job is to then distribute incoming and outgoing packets to the correct devices, and it does this by assigning each device it can see an internal IP address. Most often, you're device will have a local IP address of 192.168.1.XX, and collectively will share the external IP address your IP assigns you.

Your hardware devices can be a bottleneck. A lot of the equipment today is gigabit enabled, but it wasn't long ago, and even still true, lower grade hardware may only be 10mb throughput. Hardware can only handle X amount of data going through it at any given point, and with too many devices on the network, they can collectively exceed the bandwidth of the hardware. When that happens, or gets close to happening, decent hardware will at least have software that has QoS management, (Quality of Service), where some devices can be assigned a higher priority than others. The router may try to figure out QoS priority dynamically, which may explain why you've some devices may load assets from the internet faster than others, they're getting priority on their packets, thus they're getting a higher download/upload rate then other devices.

As far as quality of hardware, yes, it matters. For the most part, hardware is hardware, with exception whether the modem, router, or switch can provide gigabit transfer speeds. Let's say you have a gigabit switch (yay!) and it's plugged into an old modem for your IPS which doesn't support gigabit speed (sad) than the modem becomes the bottle neck, and vice versa. Another BIG factor in how performant your hardware is, is in how good, or bad the software that manages it is.

This is why enterprise grade networking equipment exists, it typical outperforms residential equipment, and the funny part is, for what high end residential equipment costs, you can typically buy entry level enterprise grade equipment for the same money, and with a little bit of knowledge and time, set that up at home and have more reliable software/hardware, that is way more configurable as well.

So you know, if you want to make sure your devices never have problems streaming Netflix, you can just give your device QoS priority, make everyone else in the house suffer with buffer times when enough bandwidth isn't available, and just smile saying you don't understand why that happens.