why do all browser lag for this given loop?

var a,b;

while (a!==b){

if (a>b) {

var a=a-b;

}

else {

var b=b-a;

}

}

so you enter numbers for a and b, and while the condition is true it is doing the given calculations and finally the a and b will became the same and the condition for WHILE will be false and after that it gives out a number a or b (doesnt matter cus both are equal already), also it works for some numbers but browser lags for some specific numbers for example if a=60 and b=120, i dont understand why :( by the way the given function calculates the largest common divisor of two given numbers, please help me and tell why is it lagging :(