MicroSD cards have been getting steadily faster over the last few years with sequential speeds that compete with 2.5-inch hard drivers and improvements in IOPS that make them more able to handle application storage. The newly announced SanDisk Extreme 400 GB is more than 50 percent faster than smaller capacity Sandisk Ultra UHS-I MicroSD cards.
https://www.notebookcheck.net/The-SanDisk-Extreme-400-GB-MicroSD-card-is-the-world-s-fastest-UHS-I-MicroSD-card.286756.0.html