Possibly due to sounds loading from disk?
I can't be sure, but I think I'm experiencing an issue where I get a big bunch of dropped frames when a sound plays for the first time.
This is most frustrating when I can get up to a full second of dropped frames during the first encounters with the enemy, but I think I've noticed framedrops on the first interaction with the armory as well.
Interestingly, the framedrops also give me a kind of 'sixth sense' where when I drop about a half second worth of frames, I can tell the enemy is nearby and alert my team! I think this may be because the sounds of aliens moving around which I might not normally hear are loading from disk.
Anyway -- I'd ask the devs to investigate the way sounds get loaded or cached and see if sound loading or playback is capable of blocking frames from rendering.
My system specs are pretty high, 8GB of RAM, 2GB of video RAM, and an SSD for my main drive, but the disk steam games lives on is an old spinner.