I thought I'd come up with a good route that landed me a 00:55, but then I checked the leaderboards and saw Fri3ndlyFir3 had already done the exact same route and got a 00:53 two years ago. With a little adjustment to methods and route, I managed to chop it down to 00:51.

The route starts by waiting out the entire cutscene at the start. If you cut it early, the guards on the first floor will spot you when you reach the top of the stairs.

I found dropping an explosive (it has to be the christmas present or it's not as thematic) faster than shooting the first target, mostly because I have terrible aim. It has to be a remote explosive as the proximity ones will not trigger until slightly too late and delay the run a couple of seconds on exit. It can be triggered once you pass the guards on the first floor.

Running the corridor is the shortest direct path to the exit, the earlier you can shoot the second target, the better. With really good aim and very fast shooting it might be possible to get a 00:50 with this route, but since it's already 15 seconds off the record, it's probably not worth it.