SharePoint 2013 Upgrade-SPSite Only Doing One at a Time

Just a word of caution if you are upgrading site collections in SharePoint 2013. Many examples will show you the following PowerShell command if you want to upgrade all your site collections:

Get-SPSite -Limit All | Upgrade-SPSite -VersionUpgrade –Unthrottled

Be wary that this will literally execute 1 site collection at a time.  By default, SharePoint will allow 10 concurrent upgrades for site collections.  If you want to take advantage of the concurrency SharePoint offers, I would recommend running the following:

$sites = Get-SPSite -Limit All | ?{$_.CompatibilityLevel -eq 14}
Write-Host $sites.Count

foreach ($site in $sites)
{
    Upgrade-SPSite -Identity $site -VersionUpgrade -QueueOnly
}

You can then monitor the progress using the following:

Get-SPContentDatabase | Get-SPSiteUpgradeSessionInfo -ShowInProgress -ShowFailed | ft

I hope this helps!