SD cards don’t usually advertise their SPI mode speed. Some cards may implement it badly or in some cases not implement SPI mode at all. This usually happens with fake and no name SD cards. I recommend Sandisk 16-32gb cards for Flippers. They fall in the sweet spot between quality and cost.
Flipper Zero works with the microSD card in SPI mode (instead of standard SDIO) so it is important to use high-quality branded microSD cards such as SanDisk, Kingston, etc. Fake cheap microSD cards can work badly in this mode.