The gauge breaks are probably a rather significant part of that. Having to offload the cargo, move it to a different set of tracks, load it back up, and then re

The gauge breaks are probably a rather significant part of that. Having to offload the cargo, move it to a different set of tracks, load it back up, and then repeat at the other end of the broad gauge tracks adds up to a rather large amount of time. I imagine you could probably shave off at least a day or two if you didn't have to deal with that part of the issue. Furthermore, there are almost certainly a number of places where you'll have to decouple and recouple the cars to a new locomotive when you cross country borders for various nationalistic and tractive reasons. Finally, you have to factor in time spent during crew breaks, crew changes, fueling the train, etc. To speed things up, you'd need to convince the former soviet republics that going to standard gauge is a good investment for increasing trade in and through their countries. Additionally, you'd need to convince various nations to sign trackage rights and operation rights agreements so you don't need to change locomotives at every border. After all that, then you can start worrying about customs agreements, which would not be that significant of a delay compared to all the other logistics of transporting things over long stretches of rail. It's just time consuming almost by definition.