PHP consumes as much resources as possible to execute script faster. So it can take 100% of one CPU, but it takes only 0.1-0.5 seconds.
So - any php script consumes some considerable amount of CPU when it is executing, but it is lasting very fast.
The reason that you see XML.php in the processes is that this file is executing more often than others (every N seconds from each user) ... since it is executing more ofter it is more likely that this file will be in processes list.
I will try see more to have a deep look at this call, to see how to optimize it, but it can not be done too much. The most useful things in this case are increase delay between calls (from 2 sec to 5 sec for example) and optimizing the server by installing some php accelerator and make sure that mysql cache is enabled.
So - any php script consumes some considerable amount of CPU when it is executing, but it is lasting very fast.
The reason that you see XML.php in the processes is that this file is executing more often than others (every N seconds from each user) ... since it is executing more ofter it is more likely that this file will be in processes list.
I will try see more