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If you can determine the first border pixel accurately eg. occurs near (0,0) then you might be able to

1. floodFill all black pixels starting at (0,0) or where ever, generate a binary image such that 0 is for border pixels, and 1 is for everything else
2. use findContour + boundingRect to find the bounding box of the inside pixels. The findContour is very fast, it only visits pixels on the edge (not inside like a floodFill). findContour will return an array of possible contours, pick the largest one.
3. crop the borders out using the boundingRect of the non-border pixels

If you can determine the first border pixel accurately eg. occurs near (0,0) then you might be able to

1. floodFill all black pixels starting at (0,0) or where ever, generate a binary image such that 0 is for border pixels, and 1 is for everything else
2. use findContour + boundingRect to find the bounding box of the inside pixels. The findContour is very fast, it only visits pixels on the edge (not inside like a floodFill). findContour will return an array of possible contours, pick the largest one.
3. crop the borders out using the boundingRect bounding rect of the non-border pixels