Is it true that windows becomes slow after the data goes beyond 75% of the drive size?
It's not necessarily true. It's not a fixed percentage--it's about how much contiguous free space you have left.
Filling up 75% of a 250GB hard drive would leave you 62.5GB left, but filling up 75% of a 1TB drive would leave you 250GB (the same amount of space as the whole first drive!)
It's true that Windows does need free space to run because you probably don't have enough memory to run the OS + all of the programs and the files you use at one time. It stores the data that exceeds the amount of available memory on the hard drive through a process called paging. This is where the free space comes in handy.
There are some other steps you can take to help keep your computer running fast at this page. One of the best ways, however, is more physical memory (RAM).
Yes, because this usually means that any non-trivial write to the hard drive (and even many of the trivial ones) will fragment. However, regular defragmenting will reduce the slowdown.