Guess the second slave chamber is more a building bug than anything else. It is not needed that early. I watched its build being started at second 45.
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Yes, when I was playing as DE, I tended on average to only build the chambers when I needed them, because I was nearing the cap limits.
But when it was time, I also made it a point to build them in pairs for the reasons I listed above. But, then I wouldn't build a second pair until I was reaching the cap limits again.
I can't remember how many it let you build, six or eight, iirc. But, with the third pair and anything thereafter, I tended to be more creative. Sometimes I used the chambers as foundations for forward bases, and other times, I simply buried them in the back areas so that I could use the extra space they generated and threw in a second barracks or second machine shop around the chambers for the DE to use. Having a producing barracks or machine shop near a chamber can provide a protection factor that's pretty much as good as having an LP2 or LP3 listening post nearby.
Hopefully Arkhan will be able to take all the input and create the optimum use for the chambers, as much as possible. As he has time to observe, I think solutions will present themselves over time.