Most of the time used seems to be caused by .NET's Process class (Access Denied exception legitimately thrown, etc.), so here is a full P/Invoke version that doesn't use it but uses the native CreateToolhelp32Snapshot function:

[DllImport("advapi32", SetLastError = true)]
private static extern bool OpenProcessToken(IntPtr ProcessHandle, int DesiredAccess, out IntPtr TokenHandle);

[DllImport("kernel32", SetLastError = true)]
private static extern IntPtr OpenProcess(int dwDesiredAccess, bool bInheritHandle, int dwProcessId);

[DllImport("kernel32", SetLastError = true)]
private static extern bool CloseHandle(IntPtr hObject);

[DllImport("kernel32", SetLastError = true)]
private static extern IntPtr CreateToolhelp32Snapshot(int dwFlags, int th32ProcessID);

[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
private static extern bool Process32First(IntPtr hSnapshot, ref PROCESSENTRY32 lppe);

[DllImport("kernel32", SetLastError = true, CharSet = CharSet.Unicode)]
private static extern bool Process32Next(IntPtr hSnapshot, ref PROCESSENTRY32 lppe);

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
private struct PROCESSENTRY32
{
    public int dwSize;
    public int cntUsage;
    public int th32ProcessID;
    public IntPtr th32DefaultHeapID;
    public int th32ModuleID;
    public int cntThreads;
    public int th32ParentProcessID;
    public int pcPriClassBase;
    public int dwFlags;

    [MarshalAs(UnmanagedType.ByValTStr, SizeConst = 260)]
    public string szExeFile;
}

public static List<List<string>> GetProcessWithUsers()
{
    var result = new List<List<string>>();
    const int TH32CS_SNAPPROCESS = 2;
    var snap = CreateToolhelp32Snapshot(TH32CS_SNAPPROCESS, 0);
    var entry = new PROCESSENTRY32();
    entry.dwSize = Marshal.SizeOf<PROCESSENTRY32>();
    if (Process32First(snap, ref entry))
    {
        do
        {
            const int PROCESS_QUERY_LIMITED_INFORMATION = 0x00001000;
            var handle = OpenProcess(PROCESS_QUERY_LIMITED_INFORMATION, false, entry.th32ProcessID);
            result.Add(new List<string> { entry.szExeFile, GetProcessUser(handle) });
            if (handle != IntPtr.Zero)
            {
                CloseHandle(handle);
            }
        }
        while (Process32Next(snap, ref entry));
    }
    CloseHandle(snap);
    return result;
}

public static string GetProcessUser(IntPtr handle)
{
    if (handle == IntPtr.Zero)
        return null;

    const int TOKEN_QUERY = 0x0008;
    if (!OpenProcessToken(handle, TOKEN_QUERY, out var tokenHandle))
        return null;

    var wi = new WindowsIdentity(tokenHandle);
    CloseHandle(tokenHandle);
    return wi.Name;
}

On my PC, I've been down from 1500 ms to 30 ms (x50).