The pads as delivered on the car need heat. The coefficient of friction is increasing with temperature. When it rains, the brake temp has tendency to be lower. The brake needs to be apply for a longer period before the temp reach the good value. The brake pads have probably a DOT spec like FG or something like. It is normal for high performance brake pads.
Elise 2006, white, K20 turbo GT3076R, 395 whp
List of mod: I am just stupid putting that much money on that car!