Hold a lump of lead in your hand. It is a flat, dark grey with no shine at all, and it is far heavier than a piece that size should be. Put it down and it stays exactly where you left it. Nearly every use of leaden hands you one of those three things: the grey, the weight, or the slowness that the weight brings.
From the word’s history
One source records leaden as Middle English leden, from Old English lēaden 'made of lead', which by surface analysis is lead + -en, the same -en as in wooden and golden. So the literal 'made of lead' sense is the oldest shape of the word, and the grey, the heaviness and the dullness are later ways of using that same picture.
Wiktionary 記錄 leaden 來自中古英語 leden、古英語 lēaden「鉛做的」，拆開就是 lead（鉛）加上 -en——跟 wooden（木製的）、golden（金製的）是同一個 -en。所以「鉛製的」是這個字最早的樣子。灰、重、悶這些意思，都是後來借用同一塊鉛的畫面。
Wiktionary 'leaden' (kaikki extract 3f6f69a190036ae4:1165148811): Middle English leden, leaden; Old English lēaden; by surface analysis lead + -en
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