Question

When I run the code after the form is initialized, it works the way I intended. But, when I load it for the first time in the form Load event it lags the UI from drawing for a second. I don’t really want to make a splash screen as there’s already some UI elements that indicate work is being done.

volatile bool ABfired = false;
    public form_Main()
    {
        InitializeComponent();
    }

    void GetSearchableDataCache()
    {
        this.UIThread(() =>
        {
            ultraActivityIndicator1.AnimationEnabled = true;
            ultraLabel_Status.Text = "Querying SQL...";
        });

        var projids = new List<string>();
        var names = new List<string>();

        ABfired = false;

        Thread A = (new Thread(() =>
        {
            while (!ABfired)
            {
            }
            projids = new sql.ProjectIDs(true).Get();
        })
        {
            Name = "Thread A", IsBackground = true
        });
        Thread B = (new Thread(() =>
        {
            while (!ABfired)
            {
            }
            names = new sql.names(true).Get();
        })
        {
            Name = "Thread B", IsBackground = true
        });

        Thread AB = (new Thread(() =>
        {
            ABfired = true;
            while (A.IsAlive || B.IsAlive)
            {
            }
            this.UIThread(() =>
            {
                ultraActivityIndicator1.AnimationEnabled = false;
                ultraLabel_Status.Text = "Ready";
            });
        })
        {
            Name = "Thread AB", IsBackground = true
        });

        A.Start();
        B.Start();
        AB.Start();
    }

private void form_Main_Load(object sender, EventArgs e)
    {
        GetSearchableDataCache();
    }

Answer 1

Your threads are spinning, so they each use up an entire CPU core.
That slows the system down dramatically.

You should never wait using a while loop like you’re doing now.
Instead, wait on a ManualResetEvent.

However, you can replace all of that code with much a simpler implementation using Tasks and no explicit threads.
Using Task.ContinueWith will allow you to run a task after the previous one finishes without wasting a thread.