I don't really think it's informal, but it can take the meaning of slightly, among other things such as a small amount, portion, or degree. It has wide usage.Is "a bit" an informal way of expressing "slightly"?
Turn it down just a bit.
Tell me a bit about yourself.
Let's spend a bit of time together this weekend.
Wait a bit and he'll show up.
I'm not too hungry but I can eat just a bit.
He's in a bit of trouble.
I'm a bit confused.
He's a bit under the weather today.
Let's chat a bit.
It's a bit off center.