I kept track of the length of my nails and kept a diary when I first started growing them long, so I can answer this from personal experience.
When my nails grew 3 mm beyond the ends of my fingers, they would frequently hit the key in the next row, causing me to have difficulty touch typing on a keyboard. For example, if I was trying to type an S, my nail would accidentally land on the W.
I preferred having long nails to typing quickly, so I continued to grow them until they were approximately 15 mm beyond the ends of my fingertips on my left hand and 12 mm on my right hand. Because of my long nails, it took me about two months to learn to type differently. I had to learn to hit the keys with the tips of my nails rather than the tips of my fingers, but I did, and now when I break a nail, I complain because I can’t type with that fingernail being shorter!
I also had to experiment with different methods to strengthen my nails, and I discovered hard gel nails to protect and build them up. Simply Organic Gel Nails, which cures under a UV lamp to become bulletproof plastic, are my current favorite product.
To type with long nails, I’ve discovered that it’s also important to keep all of my nails on the same hand extending about the same amount. It makes no difference if they are 5 mm or 15 mm long as long as they are all within 2 mm of the average length.
By the way, the shape is irrelevant. I’ve had square, round, and oval ends. I’ve had nails that curved down and nails that protruded straight from my fingertips. Once I figured out how to hit the keys with my nail tips, I could type with any shape as long as they were a consistent length across my hand.
I spend a lot of time at work on my computer, but I’ve never taken a typing test to determine my typing speed. I’m not sure, but I believe my typing speed has decreased by 15% since growing my nails because I no longer have tactile feedback from the tips of my fingers on the keys. But, as I previously stated, I value my long nails more than my typing speed, so I believe it is worthwhile!