To compare, there are more options:

import (
    "fmt"
    "regexp"
    "strings"
)

const (
    str    = "something"
    substr = "some"
)

// 1. Contains
res := strings.Contains(str, substr)
fmt.Println(res) // true

// 2. Index: check the index of the first instance of substr in str, or -1 if substr is not present
i := strings.Index(str, substr)
fmt.Println(i) // 0

// 3. Split by substr and check len of the slice, or length is 1 if substr is not present
ss := strings.Split(str, substr)
fmt.Println(len(ss)) // 2

// 4. Check number of non-overlapping instances of substr in str
c := strings.Count(str, substr)
fmt.Println(c) // 1

// 5. RegExp
matched, _ := regexp.MatchString(substr, str)
fmt.Println(matched) // true

// 6. Compiled RegExp
re = regexp.MustCompile(substr)
res = re.MatchString(str)
fmt.Println(res) // true

Benchmarks: Contains internally calls Index, so the speed is almost the same (btw Go 1.11.5 showed a bit bigger difference than on Go 1.14.3).

BenchmarkStringsContains-4              100000000               10.5 ns/op             0 B/op          0 allocs/op
BenchmarkStringsIndex-4                 117090943               10.1 ns/op             0 B/op          0 allocs/op
BenchmarkStringsSplit-4                  6958126               152 ns/op              32 B/op          1 allocs/op
BenchmarkStringsCount-4                 42397729                29.1 ns/op             0 B/op          0 allocs/op
BenchmarkStringsRegExp-4                  461696              2467 ns/op            1326 B/op         16 allocs/op
BenchmarkStringsRegExpCompiled-4         7109509               168 ns/op               0 B/op          0 allocs/op