We also have a divisibility test for 6 and 9.
For a number to be divisible by 6, it should be divisible by both 2 and 3.
like 18 is divisible by both 3 and 2 so it is divisible by 6.
For a number to be divisible by 9, its sum of digits must be equal to 9.
like 18 , here sum of the digit is 1 + 8 = 9, so it is divisible.
As far as I know and what my experience say is that you should keep practicing mathematics to make the subject simpler. With practice you will learn some techniques and even your speed of calculation will improve. Practice means continuous practice, you cannot solve one problem then go for a walk and then come back after an hour that will be a really time taking and tough process. Make a time table and try to solve as many problems a s possible in that time.I assure you you will definitely start solving sums at a much higher speed.