This is my guess too. It's a very weird design decision, especially for a developer targeted laptop. I would probably be less unhappy with a 16 GB RAM chip soldered, but than an 8 GB one.
i get that for achieving higher memory speeds, you need to often solder instead of making it available as a slot.
but most people would like to upgrade, and if it is going to most likely work on the slower speed of the second stick, it defeats the performance point altogether.
My wording is confusing, by "not taking a performance hit" I mean that the performance of the 32GB stick is not limited to the lesser 8GB stick, like it would have been with dual channel.
Of course, dual channel is always better than single channel, and it gives you a performance boost if you have 2 of the same stick.
I can not tell the configuration of the slots from their specs page [0].
Their configuration page [1] confirms that it's 8GB+32GB to get the 40GB.
Why wouldn't they just offer 2x16GB to get the dual-channel memory? Unless, of course, the laptop does not support dual-channel memory...
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[0]: https://system76.com/laptops/lemur#specs
[1]: https://system76.com/laptops/lemp12/configure