GPT vs MBR for swap devices

[bob prohaska]

In trying to get a Pi3 at r334939 to run -j4 buildworld it's become clear 
that a USB flash (Sandisk Extreme) swap device very quickly triggers OOM 
process kills, while a USB mechanical disk swap devices works perfectly. 

It's somewhat hard to believe that the Sandisk Extreme is much slower than
a mechanical hard drive for small reads and writes if the rated speeds are
anywhere close to correct, but it happens that the flash device uses MBR
partitioning, while the mechanical hard drive uses GPT.

Is there any reason to think that the partitioning scheme matters? 

Thanks for reading,

bob prohaska