I don't think it takes any longer to cum in the shower, but it definitely takes a lot more effort to cum while standing. If we're only talking about masturbation, there are several factors that could contribute.
1) Your body weight is not fully supported as it is when jerking in a bed or chair, so you have to expend extra effort for that.
2) When jerking off in standing positions, usually it's when you don't have a lot of time (i.e. the shower) and naturally you have to work harder to get off faster. Instead of taking a relaxing pace, you're squeezing your buttocks, standing on your tiptoes, jerking as fast as possible, flexing all your muscles to get yourself to cum faster, etc.