Actually read write speed depends on both the speed of the card and of the phone/device. If the phone can't support too high speeds, there's honestly no point in investing in a higher speed card, unless you probably may use it for some other device some other days.
That's true, but the App-performance is mostly determined by small blocks ( 4K read/write and 4k Q32 read/write) speed, and that varies a lot from card to card, as illustrated in this chart:
The nominal speed printed on cards are Sequential read/write speed (which is used for big blocks/files like audio, video, pictures). The bus-speed of Lumia 1520 will be the bottleneck here, but it will not be a bottleneck for small blocks (which are critical for App-performance) or medium blocks which can play a role too.
High speed cards (with high nominal/Sequential speed) tend to have much better small blocks ( 4K read/write and 4k Q32 read/write) speed too, but here there is a good deal of differences between fast cards too. Amway's, the small block speed is the most important factor for App-performance.