Okay, so you don't have an automated procedure in place yet. Understood. When you get to that point, check out the Create Network Dataset From Template tool: https://desktop.arcgis.com/en/arcmap/latest/tools/network-analyst-toolbox/create-network-dataset-fro... I think this might be more efficient and easier to do than what you're suggesting with truncating and re-populating tables.
Taking another wild guess here, which is kind of a long shot: When you make edits to the features of a network dataset, the network will create an internal table of "dirty areas" indicating which parts of the network have been changed. When you build the network, it only builds the dirty areas instead of the whole thing. This is intended to make the procedure more efficient because if you have, say a nationwide network dataset but you only edited one neighborhood, it can just leave everything except that neighborhood alone.
However, if you've made wide-sweeping edits, such as replacing all the streets with different streets in a different geography, you might have built up a sizable dirty area file. Possibly the Build process is actually slower if it's trying to sort through all these dirty areas and take care of them rather than just rebuilding the entire network as if from scratch.
But, that's just a guess which may or may not apply to your situation.
Do you get the slow behavior if you put your streets in a fresh geodatabase with a fresh feature dataset and create the network dataset completely from scratch? If not, then the above might describe what's going on. If it's still slow in this case, then something else is probably weird with your data. I would suggest you call Support so they can look at the data for you and diagnose the problem.
