The use of strconv.FormatFloat causes a string allocation,
by setting aside a reusable buffer and using strconv.AppendFloat
this can be avoided.
Before:
BenchmarkRespond-4 300 5392189 ns/op 618936 B/op 20010 allocs/op
After:
BenchmarkRespond-4 300 4713746 ns/op 139744 B/op 10 allocs/op
This benchmark is using a custom encoder that calls WriteFloat64 20k
times, which is the bulk of the work.