Searching strings with indexOf vs countUntil

[Anonymouse via Digitalmars-d-learn]

I was profiling my program with callgrind and saw that a lot of 
time was spent in countUntil (std.algorithm.searching) on 
strings. I had chosen to use it instead of indexOf (std.string), 
with the plain assumption that countUntil wouldn't decode, while 
indexOf would.

Comparing microbenchmarks between the two I see that countUntil 
is some 7x slower.


import std.stdio : writeln;
import std.string : indexOf;
import std.algorithm.searching : countUntil;
import std.datetime;

enum line = ":zorael!~NaN at asdf.asdf.asdf PRIVMSG #d :Hello 
world!";
enum iterations = 1_000_000;

void main()
{
     StopWatch sw;

     sw.start();
     foreach (i; 0..iterations)
     {
         line.indexOf(" :");
     }
     sw.stop();

     immutable usecsIndexOf = sw.peek().usecs;
	
     sw.reset();
     sw.start();
     foreach (i; 0..iterations)
     {
         line.countUntil(" :");
     }
     sw.stop();

     immutable usecsCountUntil = sw.peek().usecs;

     writeln("indexOf:    ", usecsIndexOf.usecs);
     writeln("countUntil: ", usecsCountUntil.usecs);
     writeln(cast(double)usecsCountUntil/usecsIndexOf, "x");
}


https://dpaste.dzfl.pl/0319edb79ec8 gives the output:

     indexOf:    160 ms and 534 μs
     countUntil: 1 sec, 146 ms, and 842 μs
     7.14392x

What is the fundamental difference between the two? When should I 
ever use countUntil on strings?