I noticed something interesting - turning the game off and restarting it fixed the slowdown. Is that a bug then?
Its plausible that there may be a memory leak and you finally ran out and ended up paging. Would explain the restart resolving the issue, all leaked memory is freed by the OS upon process termination.
Could be any number of other things, but that's what comes to mind first for me, and yeah, I'd think probably a bug. If it were working as intended, simply reloading should not have changed the outcome and would then be a bug.