I suggest reading this post and everything after it:
…but this could be a CPU problem or another chip-based component failing (graphics card, motherboard chipset, RAM…). Run something like HWMonitor to see what your temperatures are while PhotoLab is running slow. If the processor gets too hot, it will throttle down to low performance. You might need to reassemble the CPU-heatsink-fan stack, replace something, or clean your computer (carefully).