When you type fast, perhaps your individual taps are lighter. That would cause a keyboard to fail to register each tap if the individual keys have a stronger resistance than the pressure you are putting on each key. You might try changing positions when you type so your fingers are pressing harder on the keys. I type about 85 words per minute, and if I am touching the keys with enough pressure, the computers I have owned are always able to keep up. My first laptop was a Toshiba also, so even the older ones are able to register very fast typing. I've also owned Gateway, Samsung, and Acer laptops and frequently used component keyboards of name brands and off brands for desktop units. All are able to accommodate my 85 words per minute, though all of them have felt a little different. Each keyboard has a slightly different resistance. The only time I have had trouble was with older bluetooth keyboards. Newer ones are much better, though they still occasionally drop a letter here and there.
It could matter what word processing program you are using. If it is an older program or if your computer memory or processing is older, it may not register input as quickly. I have noticed if I am typing on an internet program, such as an answer on hubpages, if my computer memory is getting eaten up by my browser or other programs running at the same time, the typing sometimes stops registering for a moment or two. When it catches up with itself, I have to go back and make corrections. So you might consider whether your computer needs more memory or if your processor is overworked by what you have running at any given time.
Hope that helps.