You only need 30 mb/s for 4K video, which all Class-10 cards are designed to do. What you want for smartphones, contrary to cameras is a good transfer rate for 512 kb and 4 kb (kilobyte) random write (more important than read). In other words, fast transfer of those tiny files that app constantly write. People post their benchmarks on Amazon for pretty much every card available, so you can check for yourself before you decide. 4 kb random write usually goes from 0 to 3 in performance. You would believe how many high-end cards are sold (I'm talking 70mb/s sequential write and up), that can't even get a 1 in 4 kb random write. Of course the descriptions don't tell you that maybe they're not the best cards for cell phones. I personally splurged on a "SanDisk Extreme PRO 64GB UHS-I/U3 Micro SDXC Memory Card Speeds Up To 95MB/s With 4K Ultra HD Ready-SDSDQXP-064G-G46A" because the benchmarks published by customers showed 4kb random write performance above 2.3, which is really excellent.