Improve script performance

The key approach is to minimize the API calls. Performing those in for loops will slow down the process considerably, especially with 900 rows. I've re-worked your code in such a way, that it calls for the Spreadsheet API only twice:

function open_interventions() {
  var sheet = SpreadsheetApp.getActiveSpreadsheet().getActiveSheet();

  // start logger time  
  var start = new Date();

  // collect data and sort
  var data = sheet.getRange(2, 1, sheet.getLastRow()-1, sheet.getLastColumn())
    .sort([
      {column: 1, ascending: true}, 
      {column: 3, ascending: false},
      {column: 2, ascending: true}])
    .getValues();

  // set counters
  var cFirst = 0, cSecond = 0;

  // itterate to find number of "Open" rows
  for(var i=0, dLen=data.length; i<dLen; i++) {
    if(data[i][0] == 'Open') {
      cFirst++;
    } 
  }

  // itterate through array (backwards) to calculate the rowIndex 
  for(var j=dLen=data.length-1; j>=0; j--) {
    if(data[j][0] != 'Open') {      
      cSecond++;
    } else {
      break;
    }
  }

  // calculate rowIndex
  var rIndex = data.length - (cFirst+cSecond);

  // hide all rows
  sheet.hideRows(2, data.length); 

  // show (unhide) only "Open" rows
  sheet.showRows(rIndex+2, cFirst); 

  sheet.setActiveSelection("D1")
    .setValue("Execution time (ms): " + (new Date() - start).toString());
}

I've added a time tracker thingy to it, so that you can monitor the execution speed. See example file I created: Efficient Sorting. When the file is opened, it will add another menu item called Sorting. There you can play around and see what happens.

whenever you have a loop doing a delete, the first place to optimize is to see if the loop can be done backwards.

In systems like spreadsheets removing an item at the top of the sheet or the left side of the sheet is done by shifting everything to the left or top. If you have a lot of data each delete is expensive. Doing a delete starting at the bottom or the right side may be faster.

Another thing to look at is doing it as a group, deleting many rows or columns at a time, thus cutting down on the number of shifts.

to delete from the right side first change this line:

for (var i = 14; i <= numColumns - 3; i++) 

to this line:

for (var i = numColumns - 3; i >= 14 ; i--)