Sample import command that uses threading for Sitecore PowerShell Extensions.

ImportRainbowItemBulk.cs

using System.Collections.Concurrent;
using System.Collections.Generic;
using System.Linq;
using System.Management.Automation;
using System.Threading;
using System.Threading.Tasks;
using Kamsar.WebConsole;
using Rainbow.Model;
using Rainbow.Storage.Sc.Deserialization;
using Sitecore.Data;
using Sitecore.Data.Engines;
using Sitecore.Data.Events;
using Sitecore.SecurityModel;
using Unicorn.Deserialization;
using Unicorn.Logging;

namespace Unicorn.PowerShell
{
	[Cmdlet("Import", "RainbowItemBulk")]
	public class ImportRainbowYamlBulkCommand : YamlCommandBase
	{
		protected override void ProcessRecord()
		{
			if (Item == null) return;

			var cancellationToken = new CancellationToken();
			var itemsToInstall = new BlockingCollection<IItemData>();
			foreach (var newItem in Item)
			{
				itemsToInstall.Add(newItem, cancellationToken);
			}

			var threads = 8;
			var running = new List<Task>();
			for (var i = 0; i < threads; i++)
			{
				running.Add(Task.Run(() => { ItemInstaller(itemsToInstall, cancellationToken); }, cancellationToken));
			}
			foreach (var t in running)
			{
				t.Wait(cancellationToken);
			}
			itemsToInstall.CompleteAdding();
		}

		[Parameter(ValueFromPipeline = true)]
		public IItemData[] Item { get; set; }

		private void ItemInstaller(BlockingCollection<IItemData> itemsToInstall, CancellationToken cancellationToken)
		{
			Thread.CurrentThread.Priority = ThreadPriority.Lowest;

			using (new BulkUpdateContext())
			using (new EventDisabler())
			using (new SecurityDisabler())
			using (new SyncOperationContext())
			{
				while (itemsToInstall.Any())
				{
					if (itemsToInstall.TryTake(out var item, int.MaxValue, cancellationToken))
					{
						ProcessItem(item);
					}
				}
			}
		}

		private void ProcessItem(IItemData item)
		{
			if (item == null) return;

			var console = new PowershellProgressStatus(Host, "Deserialize Item");
			var consoleLogger = new WebConsoleLogger(console, MessageType.Debug);

			var deserializer = new DefaultDeserializer(false, new DefaultDeserializerLogger(consoleLogger), CreateFieldFilter());

			//consoleLogger.Info(item.Path);
			deserializer.Deserialize(item, null);
		}
	}
}

Test.ps1

$directory = "$($SitecoreTempFolder)\items"
<#
if(Test-Path -Path $directory) {
    Remove-Item -Path $directory -Recurse
}
New-Item -Path $directory -ItemType Directory

$items = Get-ChildItem -Path "master:" -ID "{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}"
foreach($item in $items) {
    $yaml = $item | ConvertTo-RainbowYaml
    $file = New-Item -Path (Join-Path -Path $directory -ChildPath "$($item.Name).yml")
    [System.IO.File]::WriteAllText($file, $yaml)
}
#>
Write-Host "Cleaning up old items"
Get-ChildItem -Path "master:" -ID "{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}" | Remove-Item -Permanently
Write-Host "Reading file data"
$yamlItems = [System.Collections.ArrayList]@()
foreach($file in Get-ChildItem -Path $directory) {
    $yaml = [System.IO.File]::ReadAllText($file.FullName)
    $yamlItems.Add($yaml) > $null
}
Write-Host "Item count $($items.Count)"
Write-Host "Importing items"
$watch = [System.Diagnostics.Stopwatch]::StartNew()
$rainbowItems = [System.Collections.ArrayList]@()

<#
foreach($item in $items) {
    $yaml = $item | ConvertTo-RainbowYaml
    $rainbowItem = ConvertFrom-RainbowYaml -Yaml $yaml
    $rainbowItems.Add($rainbowItem) > $null
}#>
foreach($yamlItem in $yamlItems) {
    $rainbowItem = ConvertFrom-RainbowYaml -Yaml $yamlItem
    $rainbowItems.Add($rainbowItem) > $null
}
Import-RainbowItemBulk -Item $rainbowItems > $null
$watch.Stop()
Write-Host "Import-RainbowItemBulk"
$watch.ElapsedMilliseconds / 1000

Write-Host "Cleaning up old items"
Get-ChildItem -Path "master:" -ID "{110D559F-DEA5-42EA-9C1C-8A5DF7E70EF9}" | Remove-Item -Permanently
Write-Host "Importing items"
$watch = [System.Diagnostics.Stopwatch]::StartNew()
foreach($yamlItem in $yamlItems) {
    $rainbowItem = ConvertFrom-RainbowYaml -Yaml $yamlItem
    Import-RainbowItem -Item $rainbowItem > $null
}
$watch.Stop()
Write-Host "Import-RainbowItem"
$watch.ElapsedMilliseconds / 1000