ToME: the Tales of Maj'Eyal

Is there an addon that reduces sight?

Having a wide sight is an advantage, but it can be inconvenient at times.

It makes the game slow. I just want to rush.

But I want to use items like focus lens.

I think increasing the sight is a cheat, but reducing the sight is QOL.

Is there any addon that can reduce the sight by inputting the number?

Equipping a Duathedlen Heart could solve any "excess vision" problems.

However you talk about the game being slow on your system. For most people, poor games performance is due to their graphics adaptor rather than CPU, so have you tried reducing/disabling graphics effects (or even running in ASCII mode) to see if that helps?