25ms is a lot of time. The Arduino is definitely fast enough. This looks like an issue with you camera.
I'd say 25ms is quite fast for a digital camera - probably does last minute focus check and then powers a servo to set the aperture before enabling the CCD. If you can arrange to manually set max aperture that might help, but I'd pretty certain responding to lightning detectors was not a design requirement for even a top flight DSLR!
Worth hunting through the manual to see if there are other clues. Unfortunately lightning detectors give no advance warning since radio and light waves travel the same speed!