Browser performance?

Question

JavaScript

I decided to find out which of the ways to draw a tile map would be faster - once to draw the entire map into a texture, and then draw it - or to form the necessary piece of tiles every frame. While writing a test program, I got a strange result.
The program creates a main canvas, 640x480, and a texture canvas - 10 times larger. The test function draws the texture on the main canvas many times, in random coordinates. It is called 5 times in a row, after which there is a pause for 5 seconds. The question is why the same function is executed more and more slowly from time to time? This is hardly noticeable in firelis, but very clearly visible in chrome - the execution time of the 1st and 5th times differs by 10 times.
Demo