use the strconv package

docs

strconv.FormatBool(v)

func FormatBool(b bool) string FormatBool returns "true" or "false"
according to the value of b

You could see the source code of ParseBool here. It uses switch too but with more cases. If the compiler inlines its code it, speed should be very similar to your code. If you want a definite answer you should benchmark different cases.

In general I discourage you from paying attention to this small details. It's just matter of some nano seconds but it make your codes more difficult to understand. Begin optimizations with profiling your code to find hotspots that take a lot of time and fix them.

Int to bool is easy, just x != 0 will do the trick. To go the other way, since Go doesn't support a ternary operator, you'd have to do:

var x int
if b {
    x = 1
} else {
    x = 0
}

You could of course put this in a function:

func Btoi(b bool) int {
    if b {
        return 1
    }
    return 0
 }

There are so many possible boolean interpretations of integers, none of them necessarily natural, that it sort of makes sense to have to say what you mean.

In my experience (YMMV), you don't have to do this often if you're writing good code. It's appealing sometimes to be able to write a mathematical expression based on a boolean, but your maintainers will thank you for avoiding it.

The simplest way would be to use the strconv.ParseBool package. Like this:

func (b *NullBool) UnmarshalJSON(data []byte) error {
    var err error
    b.Bool, err = strconv.ParseBool(string(data))
    b.Valid = (err == nil)
    return err
}