when open big excel file, it cost much time in create and release object, How to reduce time consumption

[rhddlr]

version: [0461eac]

// create object
const std::unique_ptr<QXlsx::Document> xlsx = std::make_unique<QXlsx::Document>(filePath);
// write some cell
// save object
xlsx->saveAs(filePath);
// release

when open 200KB excel file: create object: 2106ms call saveAs() func: 3450ms release object: 3500ms

[rhddlr]

test code for it, but i dont know how to solve it. https://github.com/QtExcel/QXlsx/compare/master...rhddlr:QXlsx:test_issue236