Seen while it was taking a long time:
in System.Drawing.Rectangle:get_Location () <0x2d>
in System.Drawing.Rectangle:op_Equality (System.Drawing.Rectangle,System.Drawing.Rectangle) <0x13>
in System.Drawing.Rectangle:Contains (System.Drawing.Rectangle) <0xc0>
in System.Windows.Forms.Hwnd:AddInvalidArea (System.Drawing.Rectangle) <0xbc>
in System.Windows.Forms.Hwnd:AddInvalidArea (int,int,int,int) <0x57>
in System.Windows.Forms.XplatUIX11:AddExpose (System.Windows.Forms.Hwnd,bool,int,int,int,int) <0x31>
in System.Windows.Forms.XplatUIX11:Invalidate (intptr,System.Drawing.Rectangle,bool) <0xc6>

Then this is my instrumented code:
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=16,Width=16,Height=8}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=16,Height=93}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=16,Width=16,Height=8}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=16,Width=16,Height=8}
Rectangle count: 1
processing 0 System.Drawing.Region {X=0,Y=0,Width=117,Height=93}
Rectangle count: 4999
processing 0 System.Drawing.Region {X=0,Y=116,Width=17,Height=21}
processing 1 System.Drawing.Region {X=0,Y=174,Width=17,Height=21}
processing 2 System.Drawing.Region {X=0,Y=232,Width=17,Height=21}
processing 3 System.Drawing.Region {X=0,Y=290,Width=17,Height=21}
processing 4 System.Drawing.Region {X=0,Y=348,Width=17,Height=21}
processing 5 System.Drawing.Region {X=0,Y=406,Width=17,Height=21}
processing 6 System.Drawing.Region {X=0,Y=464,Width=17,Height=21}
processing 7 System.Drawing.Region {X=0,Y=522,Width=17,Height=21}
processing 8 System.Drawing.Region {X=0,Y=580,Width=17,Height=21}
processing 9 System.Drawing.Region {X=0,Y=638,Width=17,Height=21}
processing 10 System.Drawing.Region {X=0,Y=696,Width=17,Height=21}
processing 11 System.Drawing.Region {X=0,Y=754,Width=17,Height=21}
processing 12 System.Drawing.Region {X=0,Y=812,Width=17,Height=21}
processing 13 System.Drawing.Region {X=0,Y=870,Width=17,Height=21}
processing 14 System.Drawing.Region {X=0,Y=928,Width=17,Height=21}
processing 15 System.Drawing.Region {X=0,Y=986,Width=17,Height=21}
processing 16 System.Drawing.Region {X=0,Y=1044,Width=17,Height=21}
processing 17 System.Drawing.Region {X=0,Y=1102,Width=17,Height=21}
processing 18 System.Drawing.Region {X=0,Y=1160,Width=17,Height=21}
processing 19 System.Drawing.Region {X=0,Y=1218,Width=17,Height=21}
processing 20 System.Drawing.Region {X=0,Y=1276,Width=17,Height=21}
processing 21 System.Drawing.Region {X=0,Y=1334,Width=17,Height=21}
processing 22 System.Drawing.Region {X=0,Y=1392,Width=17,Height=21}
processing 23 System.Drawing.Region {X=0,Y=1450,Width=17,Height=21}
processing 24 System.Drawing.Region {X=0,Y=1508,Width=17,Height=21}
processing 25 System.Drawing.Region {X=0,Y=1566,Width=17,Height=21}
processing 26 System.Drawing.Region {X=0,Y=1624,Width=17,Height=21}
processing 27 System.Drawing.Region {X=0,Y=1682,Width=17,Height=21}
processing 28 System.Drawing.Region {X=0,Y=1740,Width=17,Height=21}
processing 29 System.Drawing.Region {X=0,Y=1798,Width=17,Height=21}
processing 30 System.Drawing.Region {X=0,Y=1856,Width=17,Height=21}
processing 31 System.Drawing.Region {X=0,Y=1914,Width=17,Height=21}
processing 32 System.Drawing.Region {X=0,Y=1972,Width=17,Height=21}
processing 33 System.Drawing.Region {X=0,Y=2030,Width=17,Height=21}
processing 34 System.Drawing.Region {X=0,Y=2088,Width=17,Height=21}

Now a few iterations later:
processing 771 System.Drawing.Region {X=0,Y=44834,Width=17,Height=21}
processing 772 System.Drawing.Region {X=0,Y=44892,Width=17,Height=21}
processing 773 System.Drawing.Region {X=0,Y=44950,Width=17,Height=21}
processing 774 System.Drawing.Region {X=0,Y=45008,Width=17,Height=21}
processing 775 System.Drawing.Region {X=0,Y=45066,Width=17,Height=21}
processing 776 System.Drawing.Region {X=0,Y=45124,Width=17,Height=21}
processing 777 System.Drawing.Region {X=0,Y=45182,Width=17,Height=21}
processing 778 System.Drawing.Region {X=0,Y=45240,Width=17,Height=21}
processing 779 System.Drawing.Region {X=0,Y=45298,Width=17,Height=21}