V. long load times for first mission in a session?

The first time I play a campaign mission during a gaming session, it takes around 3 minutes to load between leaving the campaign select menu and actually starting the level.

The appropriate mission screen shows up and the loading circle spins, but the progress bar stays completely empty for around 2 and a half minutes, before rapidly filling up in the last 30secs and the level starting. Subsequent levels load quickly (within a few seconds, and with constant progress bar activity); it’s only the first level in a session that takes a long time to load.

I can understand if it’s something to do with loading the engine into memory, but I wanted to check and see what the expected load times should be.
In case it has some bearing on the load times:

  • MCC is installed to the internal hard drive (I have a 2tb external WD Passport drive hooked up via USB 3.0, but don’t want to mess around with switching the game over to the external drive if it’s not going to make a difference) - I’m playing local split-screen co-op with my wife; we’re both signed in before we play, and I have an Xbox Live Gold / paid account whereas hers is Silver / unpaid - I haven’t tried online multiplayer at all, so I have no idea how long the load times are for that - Other games installed on the internal drive (Killer Instinct, Rare Replay, Titanfall) load reasonably quickly
    TL;DR: The first level I play after starting MCC takes 3 mins to load. Subsequent missions take about 20-30 secs. Is this loading behaviour normal?

I’m not sure what the source of the problem was, but having moved all game files to the external hard drive, loading times are now around 30secs even when playing the first campaign mission in a session.