Will gotk3 be fast enough for a good user experience?
Yes. Unless you write a very specific program like for low latency trading, the language in which you write your app does not matter.
Just pick up the language you prefer: Go, Rust, C, Python, D, etc. they are all used in various GNOME application without trouble.
The thing that matters is the algorithm you use. If you use an O(n²) instead of an O(log n) algorithm for example, it will be bad in C, Rust and Go.