Tennis effort and intensity varies by point and by situation. It's impossible to maintain peak effort for 300 straight points or something. Yes, there is such a thing as clutch where players produce outlier performance in big moments, but I actually question how much of it is real. Much of it is probably attributable to consistently raising your effort and intensity in the biggest moments, which means you had that level in the first place (i.e. superiority). I think clutch vs unclutch is related to mentality and courage which may stop players from attaining a level they have the capacity to produce in those big moments. Sometimes it stops players from the very first ball, sometimes it stops them in the bigger points. But besides that, much of it is basically who can do the most damage when they are completely locked in and moving at full intensity, which you may not get an accurate accounting of for most of the match (for the truly great ones you usually do though which is what makes them so great, and it's in these matches that true clutch can be exhibited).