my game freeze when arriving at a specific level at the first trigger

I made a game of 4 levels. the 3 levels run smoothly. however, at the 4th level, when my character hit the first trigger, the game freeze for a moment then continue normally. that happens when I play the game for the first time in a while. if I play it again and again the freeze time is reduced but there is always a delay when the character hits the first trigger. the freeze thing happens every time I export the game. if I run the level in the unity editor, the level runs smoothly. what is the cause of it and how can I fix it?