速 (quick) +
溶 (dissolve) — used as a modifier on consumable powders:
速溶咖啡 instant coffee,
速溶奶粉 instant milk powder,
速溶汤 instant soup. Don't confuse with
速食 sùshí 'fast food' or
即食 jíshí 'ready-to-eat' —
速溶 specifically describes powders meant to dissolve in liquid.