Why hangs LibreOffice Calc for several minutes with 100% CPU load when opening, modifying or saving a spread sheet?

I struggled that last two days with the above problem:

  1. I had a spread sheet a was happily working with.
  2. At some point I made a change that caused calc not responding for several minutes when opening, modifying or saving it.

I looks like calc steadily hanging in the first place. However after serveral (10 - 15) minutes of high CPU load it came back - until the next modification.
=> I was not able to work with this spread sheet.

I did not find a solution in the web.

I now found the reason for the problem and a way to fix it.:

  1. I somehow managed to make my spread sheet having a column (L) with 1048576 lines and every 2nd cell in the column having a border line. No idea how I managed to do this. I did not do it manually. Copying overlapping areas?
  2. I was able to scroll down the 1048576 lines with the slider at the side. Normally it ends close to the last line witch contents. So it looks as if I had content in the cells. However marking the whole table and deleting any contents did not help.

I got rid of the problem by doing one of the following:

  1. Deleting the column L having the cells with borders.
  2. Marked all lines which should not have contents and removed contents and all direct formats.

If this is problem LibreOffice can/wants to address I can provide a spread sheet.