VHDL is a Hardware Description Language. It is used to describe the behaviour and structure of a programmable logic chip, such as a CPLD or a FPGA.
It is similar to Verilog, which is mainly used in the USA, whereas VHDL is more common in Europe.
When speed is essential, a protocol can be implemented in such a component, rather than by software running on a MCU.
VHDL can be unit tested to guarantee behaviour, just like C/C++ code. There are also comprehensive simulation environments that can be used to validate timing and performance.