The new 512GB high-performance 633x microSDXC UHS-I from Lexar is mainly designed for (Android) smartphones. It is specified as A2 and should therefore allow fast app starts (4000 IOPS random read, 2000 IOPS random write). However, it should also be suitable for 4K recording with action cams or drones, since the Video Speed Class 30 (V30) standard is also supported. The max. reading speed is given with 100MB/s, data are written according to manufacturer with up to 70MB/s. Thus the speeds of the SanDIsk Extreme 400 GB UHS-I microSDXC card are not quite reached.
The Lexar High-Performance 633x microSDXC should already be available and cost 260 Euro.