Its a page fault problem

In computer storage, Bélády's anomaly is the phenomenon in which increasing the number of page frames results in an increase in the number of page faults for certain memory access patterns. This phenomenon is commonly experienced when using the First in First Out (FIFO) page replacement algorithm.

Also called FIFO anomaly.

Usually, on increasing the number of frames allocated to a process' virtual memory, the process execution is faster, because fewer page faults occur.

 Sometimes, the reverse happens, i.e., the execution time increases even when more frames are allocated to the process. This is Belady's Anomaly. This is true for certain page reference patterns..