Explanation about SPI speed

Hello , I started to using yours library. I am impressed with how many things I have achieved on my Raspberry PI with this library. I created a device which comunicate with sensor throught SPI. But there is one isue which I can't solve. It's about SPI speed, while I am using class **SoftwareSpi** I am achiving sample rate around 40k samples/s. When I am trying to use your class **SpiDevice** which from my understanding is destined to use hardware SPI my sample rate is around 20k samples/s. Can you explain me what its caused by? From the definition hardware SPI should be faster than software. It's there any solution to achive faster SPI speed while using
hardware SPI ? Can you give more detail's about SPI speed in the documentation's. Thanks for your's answer.