similar problem used to occur in older pentium-era computers back in the day. A motherboard may say it could handle 4 gig ram, but it ran faster with 1 gig.

Yeah, those were the times ... ;-)

The actual reason was that the mainboard + chipset combination was only able to cache the first 2**n bytes of memory, the remaining memory was accessed uncached. On some mainboards, one could add a tag RAM chip, for an additional 2**n bytes of cached memory, but often that still was factor 2 or 4 less than the maximum memory size.

DOS and ancient Windows ran in the lower parts of memory, whereas modern operating systems use the full memory range, often one half of the addressable space for applications and the other half for the OS. On such a mainboard, either application or OS would run uncached, assuming a linear memory map from virtual to physical memory.

Another but similar effect could be observed with EMS, where "high" memory was mapped into a small window in "low" memory, first in hardware, later using x86 protected or virtual mode. Those days, "high" memory started at 1 M. Now, we have a similar barrier at 2**32 bytes = 4 Gbytes, and tricks similar to EMS are used to make use of "high" memory behind the 4 Gbytes barrier. The only way to get back to a "flat" memory model is to switch the CPU and the OS to 64 bit mode.

So, I would look at the hardware specs to learn if there is a limit for cacheable memory (I doubt it, because caching is now done inside the CPU case, no longer on the mainboard), and I would check that the OS is running in 64 bit mode.

Alexander