I had slow load times for one of my programs. I decided to see how much time was spent on the server executing the PHP code. I did this by writing the system time to a file at the beginning of the script and then again to a separate file at the end of the script, and I subtracted the start time from the end time. Note, I wrote the time to file because I was using AJAX and a print statement would break the program, so if your not using AJAX then you can use print statements.
My program was taking up to 10 seconds executing the PHP code. I then output the system time in various points in the code, and repeated the process until I was able to narrow it down to one small section of code. I fixed the code and got the load time to less than a second.
Hope this helps