Which DOM operation typically has the highest performance cost?
Forcing layout recalculation (reflow) has the highest performance cost among DOM operations because it: 1) Requires recalculating positions and dimensions of elements, 2) Can trigger a cascade of additional calculations affecting child and parent elements, 3) Blocks other browser operations while executing, 4) Often triggers subsequent repaints. Common causes include: reading layout properties (offsetWidth, clientHeight), changing element dimensions, adding/removing elements, and changing the viewport size. To optimize performance, minimize forced layouts and batch necessary layout calculations.