Windows 10: throw old ram buy new ram does?

In a nutshell, no. RAM doesn't "wear out" or get slower as it ages.

You can add more RAM, but if you already have 16GB, unless you're doing heavy gaming, heavy graphics or maybe music, you have enough.

At this point, the only way to more speed is to buy a newer, faster computer.

Like my mechanic brother would say, "Jack it up and run a new one under it!"

 

The main point of more RAM is that it reduces swops to another slower drive, be it an HDD or SSD.
So the overall performance will be better.

Speed is the wrong word to use. As the extra RAM will be running at the same speed, as indeed will other components.

RAM does not get slower with age. You can shorten it's life by overclocking and exceeding it's design ratings.

RAM does not work like you think it does, you will have to look that one up. If you are non-technical rather hard to follow how it actually works.

As for MOBO RAM slots refer to your manual for details.