Will multiple SPI slaves lower data transmission?
Since SPI is a bus, the bus is shared between all connected components. With a single bus, the communication at any time is limited to a single pair of SPI master and slave. It's not possible to communicate with two slaves concurrently.
A single bus will have a lower transmission rate compared to two or more SPI buses working concurrently.
However, if you your code does not take advantage of concurrency (i.e. is not neither using DMA transfers nor multiple tasks in an RTOS), it will only communicate on a single SPI bus at at any time. In that case, the data rate of a single bus is the same as multiple buses.
In short:

If your code can take advantage of concurrency, multiple SPI buses offer higher data bandwidth.
If your code does not take advantage of concurrency, multiple SPI buses offer no advantage.

Also check if the SPI bandwidth is really a limiting factor. If not, there is nothing to be gained with complex code and multiple SPI buses.