Speeding up algorithm

I have a simulation of bouncing balls of different sizes in a Canvas. The properties of every ball is in a class Circle and I have an array of type Circle to store the whereabouts of the circles.
A timer controls the movement of the circles (velocity) and I check collisions of the circles via a math formula ( Collision Detection (jeffreythompson.org)). For a very smooth operation I use 20/1000 of a second as timer tick. The Canvas is repainted in this timer tick as well.

The collision detection uses 2 intertwined loops to check every circle against the position of every other circle. So amount of loops = circles^2. I noticed that lowering the timer interval to 1 does not really affect the speed as (I think) the loop cannot be completed within 1 timer tick with circles = 500. Any suggestions to speed up the algorithm?

Public Sub CheckCollision()
  
  For i As Integer = 0 To UBound(Circles)
    'Bounce away from Canvas edges
    BoundariesCheck(Circles(i))
    
    For j As Integer = i + 1 To UBound(Circles)
      Dim dx As Double = Circles(j).x - Circles(i).x
      Dim dy As Double = Circles(j).y - Circles(i).y
      Dim distance As Double = Sqrt(dx * dx + dy * dy)
      
      If distance <= (Circles(i).radius + Circles(j).radius) Then
        'There is a collision as radius of both circles touch each other
        'Update statistics
        Collisions = Collisions + 1
        Circles(i).HadCollisions = Circles(i).HadCollisions + 1
        Circles(j).HadCollisions = Circles(j).HadCollisions + 1
        
        'Calculate the angle of collision
        Dim angle As Double = ATan2(dy, dx)
        
        'Calculate the overlap distance
        Dim overlap As Double = (Circles(i).radius + Circles(j).radius) - distance
        
        'Move Circles apart based on the overlap
        Circles(i).x = Circles(i).x - (overlap / 2) * Cos(angle)
        Circles(i).y = Circles(i).y - (overlap / 2) * Sin(angle)
        Circles(j).x = Circles(j).x + (overlap / 2) * Cos(angle)
        Circles(j).y = Circles(j).y + (overlap / 2) * Sin(angle)
        
        'Store current velocities
        Dim v1x As Double = Circles(i).velocityX
        Dim v1y As Double = Circles(i).velocityY
        Dim v2x As Double = Circles(j).velocityX
        Dim v2y As Double = Circles(j).velocityY
        
        'Calculate the Masses based on radius (larger radius = greater Mass)
        Dim Mass1 As Double = Circles(i).radius
        Dim Mass2 As Double = Circles(j).radius
        
        'Calculate new velocities based on Mass
        Circles(i).velocityX = (Mass1 - Mass2) / (Mass1 + Mass2) * v1x + (2 * Mass2) / (Mass1 + Mass2) * v2x
        Circles(i).velocityY = (Mass1 - Mass2) / (Mass1 + Mass2) * v1y + (2 * Mass2) / (Mass1 + Mass2) * v2y
        
        Circles(j).velocityX = (2 * Mass1) / (Mass1 + Mass2) * v1x + (Mass2 - Mass1) / (Mass1 + Mass2) * v2x
        Circles(j).velocityY = (2 * Mass1) / (Mass1 + Mass2) * v1y + (Mass2 - Mass1) / (Mass1 + Mass2) * v2y
        
        'Ensure velocities are not too small
        If Abs(Circles(i).velocityX) < 0.1 Then Circles(i).velocityX = 0.1 * Sgn(v1x)
        If Abs(Circles(i).velocityY) < 0.1 Then Circles(i).velocityY = 0.1 * Sgn(v1y)
        If Abs(Circles(j).velocityX) < 0.1 Then Circles(j).velocityX = 0.1 * Sgn(v2x)
        If Abs(Circles(j).velocityY) < 0.1 Then Circles(j).velocityY = 0.1 * Sgn(v2y)
      End If
    Next
  Next
  
  
End Sub

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