Just to be clear - the advantage of the SuperPID is that it reads the RPMs and adjusts the power to the motor to achieve your desired spindle speed. Everything else mentioned here simply tells you the speed of your spindle. So, for instance, you adjust your speed according to your homemade Arduino tachometer, then once is starts cutting and the bit is under load, the spindle speed will decrease substantially.
The SuperPID adjusts the power to the motor to account for the load so your spindle speed remains consistent - even under load. It is an ACTIVE speed controller. Whereas the stock software with the x-carve proportionally applies power to the spindle but there is no feedback mechanism - so your actual spindle speed is anyone’s guess.
So, you really do not have a spindle speed controller without a feedback mechanism.
Interestingly, the longer you CNC, the less important this feature becomes because you will start to “hear and feel” a sweet spot for a given bit in a given job on a given machine and you will want precise manual control.
(unless you are using commercial CNC machines which have incredible torque and active spindle speed mechanisms coupled with variable frequency drives).
Reading through this forum, there is a strong tendency toward using software to determine feeds, speeds and spindle rpms. You will break a lot of bits this way. This machine and firmware is just not capable of this level of control. Every machine is different, every bit and the material you are cutting is different.
Just my honest opinion. I have been building home made CNC machines for only ten years but each modification brings a new set of capabilities and variables. That is the most gratifying element of this time and money consuming hobby! Have fun.