I wonder if there is a (probably theoretical) argument that a machine could go slower with more memory sticks? - in theory at least the more-populated DRAM bank would run hotter because there's less airflow. I don't think DRAM thermally throttles like CPU/GPUs do, so it's probably not a real effect. But it's a thought.
Many platforms default to lower memory clocks as number of ranks of memory per channel increases. Sometimes these reductions are significant.
DRAM doesn't typically thermal throttle the way a CPU, GPU, or SSD does, but they do have temperature dependent refresh intervals at the more extreme ends of operating temperature. For example, past 85C most DRAM is specified to refresh twice as often, which does reduce performance because more time is spent refreshing rather than accessing data.
Real world. If all you needed was 16Gb, for example, downgrading from 3200Mhz ram to 2333Mhz RAM in order to stretch to 32Gb might actually be counter-productive.
Yes, and clock speed isn't everything either.
My 5820K system, for example, has the fastest memory that is practical for me to run in it 24/7. It's a batch of old Micron ICs that will do 2667 with very tight timings. I could potentially find the same ICs on some double sided memory, but the extra ranks would mean more strain on the memory controller and hotter running DIMMs, meaning it would be unlikely I'd be able to run them at the same timings. I could also buy memory capable of higher clocks, but the platform does not handle higher clocks well, and I'd have to loosen timings so much that most DDR4-3200 would be slower than the memory I'm using at DDR4-2667.
On my new Ryzen 3900X SFF system, I wanted 64GiB of memory and only had two DIMM slots to work with. I sacrificed considerable memory performance and infinity fabric performance for the extra density. I was able to get much better than stock performance out of the OEM Samsung sticks I purchased (DDR4-2667 CL20 that I run at 3266 CL17), but the same amount of money could have bought me 16 or 32GiB of DDR4-3600 or 3800 at CL14 to 16.