PowerShell String Timing

In this case, I would create an array of TimeSpan objects from the strings in $content and use these for the output.

IF $content is an array, use it straight away.
IF $content is a single string, you need to split it first with $content = $content -split '\r?\n'

# assuming $content is a single string, so we need to split into array:
$Content = @"

[Step1]::14:37:11
[Step2]::14:39:11
[Step3]::14:42:14
[Step4]::14:45:09
[Step5]::14:57:12
"@ -split '\r?\n'

Next loop through this string array and remove empty lines if there are any:

$times = $content | Where-Object { $_ -match '\S' } | ForEach-Object {
    $h, $m, $s = ($_.Trim() -replace '.*(\d{2}:\d{2}:\d{2})$', '$1') -split ':'
    [TimeSpan]::new([int]$h, [int]$m, [int]$s)
}

for ($i = 1; $i -lt $times.Count; $i++) {
    $timeDiff = $times[$i] - $times[$i - 1]
    # output the times taken for each step
    "Step $($i + 1) took $($timeDiff.ToString()) to complete."
}

Output:

Step 2 took 00:02:00 to complete.
Step 3 took 00:03:03 to complete.
Step 4 took 00:02:55 to complete.
Step 5 took 00:12:03 to complete.

Regex details for -replace:

.            Match any single character that is not a line break character
   *         Between zero and unlimited times, as many times as possible, giving back as needed (greedy)
(            Match the regular expression below and capture its match into backreference number 1
   \d        Match a single digit 0..9
      {2}    Exactly 2 times
   :         Match the character “:” literally
   \d        Match a single digit 0..9
      {2}    Exactly 2 times
   :         Match the character “:” literally
   \d        Match a single digit 0..9
      {2}    Exactly 2 times
)
$            Assert position at the end of the string (or before the line break at the end of the string, if any)

$Content = @"

[Step1]::14:37:11
[Step2]::14:39:11
[Test3]::14:42:14
[Example4]::14:45:14
"@ -split '\r?\n'

$times = $content | Where-Object { $_ -match '\S' } | ForEach-Object {
    # capture the action like '[Example4]' and the timestamp part
    $action, $processTime = $_.Trim() -split '::'
    # split the timestamp into Hours, Minutes, Seconds 
    $h, $m, $s = ($processTime -replace '.*(\d{2}:\d{2}:\d{2})$', '$1') -split ':'
    # output an object with two properties, the action name and a TimeSpan
    [PsCustomObject]@{
        'Action' = $action
        'Time'   = [TimeSpan]::new([int]$h, [int]$m, [int]$s)
    }
}

# loop through the returned array, get the time difference between steps
for ($i = 1; $i -lt $times.Count; $i++) {
    $timeDiff = $times[$i].Time - $times[$i - 1].Time
    # output the times taken for each step, in this new requirement rounded to show minutes only
    '{0} took {1} minutes to complete' -f $times[$i].Action, [math]::Round($timeDiff.TotalMinutes, 0)
}

Output:

[Step2] took 2 minutes to complete
[Test3] took 3 minutes to complete
[Example4] took 3 minutes to complete