Start optimizations

[CharlesNepote]

• prefer let and const vs var
• batch DOM changes; https://jonchristopher.us/blog/a-couple-of-quick-tips-for-javascript-optimization/
• prefer document.getElementById() when possible (for performance)
• prefer vanilla JS to Jquery when too costly in terms of readability; see https://tobiasahlin.com/blog/move-from-jquery-to-vanilla-javascript/
  • use jq() instead of $(), to clearly identify where jquery is used?
• eventually replace jquery with small shortcuts such as in https://stackoverflow.com/questions/6398787/javascript-shorthand-for-getelementbyid
• launch script at document start? see https://stackoverflow.com/questions/4978736/how-to-run-greasemonkey-script-before-the-page-content-is-displayed