The 20MHz often spec'ed when buying the chip is the maximum speed. Actual speed it will run at depends on what you use for a clock source (if crystal, the speed of said crystal).
When you do Burn Bootloader, you should be using the same ISP programmer, wired the same way, that you used to upload a sketch via ISP. Burning bootloader can only be done via an ISP programmer. There's no way to burn bootloader through the serial connection. It looks to me like you're not doing that?