Not true. The slowness is only due to the constant Windows updates being applied during startup. My desktop with HDD booted up more than fast enough when W10 came out. Outside of startup things are more than quick enough. But I don’t have PC on all the time and it often whilst it’s applying the updates that I want to be doing something.
It is your opinion that they are "quick enough", and
for you, that may be true.
But that certainly doesn't make what he said untrue at all. There is absolutely no doubt at all,that if you are looking to improve the performance of your PC,and it doesn't currently have one, then adding an SSD and moving the OS and programs to it very much gives the best bang for the buck. The fact
you don't feel the need for faster startup doesn't negate that point.
And you are completely wrong about the slowness being "only due to the constant Windows updates being applied during startup".
That has an effect every now and then. Probably once every 2-3 weeks on average, and it isn't all that big an inconvenience. It is trivial compared to the gains from an SSD.