Work out spi clock speed instantly with clear inputs, formula shown and shareable results.
SPI clock is the peripheral clock divided by the prescaler, and transfer time is bits divided by that rate plus per-transfer overhead. Overhead matters: at 6 MHz a 512-byte block takes 683 microseconds so 4 microseconds of setup is negligible, but for single-byte transfers it dominates completely.
SPI timing
SCK = f / prescaler; transfer time = bytes x 8 / SCK + overhead
Trace length and capacitance, the slave's maximum clock rating, and round-trip delay on longer buses. Above about 20-30 MHz signal integrity becomes the constraint.
It removes per-byte CPU overhead, which is what actually caps throughput on small transfers and frees the core for other work.