Wi-Fi performance is a complex subject. We can't explain it all there, but here are a few important things to be aware of when shopping for a new router. How To Buy A Wireless Router - 2017 Edition also has more helpful information.

• In general, the numbers you see in Wi-Fi product descriptions are greatly inflated and bear no resemblance to the performance you will get in real life. Numbers like N900 and AC5300 are product classes. Classes represent a total of the individual link rates for each radio and indicate the Wi-Fi standard used. Wi-Fi Classes are decoded in this article.
• Link rate is the theoretical maximum capacity of a network connection. This is the rate that bits could move if everything were perfect. Your actual throughput will never equal this number for many reasons. Link rate is what people typically refer to as "speed". It is the number you will see reported by Wi-Fi utilities and connection properties. It is not an actual measure of performance, but determined by how a connection is negotiated by the router and device.
• Throughput is the rate data actually travels across your network. It is what determines how long it will take to download that huge file and whether the video you're trying to watch will be smooth or choppy. Think of it as the "bottom line" for performance. The only way you can determine it is by measuring it. Manufacturers never talk about actual throughput because they can't. There are too many variables.
• The relationship between link rate and throughput is complicated. When signals are strong, higher link rate usually means higher throughput. But as signal levels drop, sometimes higher throughput is achieved by a lower link rate.
• The maximum throughput you'll get from a wireless router depends both on its class rating and the class rating of the devices used with it. The general rule is slowest device wins in determining maximum link rate. This article explains how to determine maximum link rate from a combination of router and device classes.
• To achieve maximum performance, device class must equal router class. However, due to size, power consumption and cost reasons, device radios are usually limited to lower classes than routers. Today's devices are generally AC1200 class at best, with many still N150 or AC580. This is why buying a router with a bigger number on the box usually doesn't increase performance proportionally. So you won't see a 2x increase in throughput by upgrading from an AC1200 router to an AC2600.
• Pay no attention to coverage claims. A claim of "coverage up to 14,000 sq. ft." means nothing, which you'll see if you check the disclaimer footnote.