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 'fast food' or
即食 'ready-to-eat' -
速溶 specifically describes powders meant to dissolve in liquid.