var a = "foo bar foo";
var b = "foo";

int numTimes = 1000000;

var swOrdinal = Stopwatch.StartNew();

for (int i = 0; i < numTimes; i++)
    a.StartsWith(b, StringComparison.Ordinal);

swOrdinal.Stop();

Console.WriteLine("Ran {0:N0} ordinal iterations in {1}ms", numTimes, swOrdinal.Elapsed.TotalMilliseconds);

var swCulture = Stopwatch.StartNew();

for (int i = 0; i < numTimes; i++)
    a.StartsWith(b);

swCulture.Stop();

Console.WriteLine("Ran {0:N0} culture-sensitive iterations in {1}ms", numTimes, swCulture.Elapsed.TotalMilliseconds);

Just an example of timing code

Timing different variants of string.StartsWith

C#