If you have CPU affinity happening either explicitly or implicitly due to the 40-core limitation that could have some affect on IO throughput.

However, the MOST common cause of IO slowness is simply too-few spindles for the IO requests being made (often in combination with too little RAM as well). The next most common cause is piling all kinds of batch-type activities on top of each other. Can't tell you the number of times I do a performance review at a client and see 4, 7, dozen+ jobs or batch activities all set to start at 00:00, or 02:00, etc. SQLJOBVIS is a free tool that can show you a gantt chart of your sql agent jobs to help detect overlaps. Do be sure to check external processes that could be happening too.

There are MANY other things that could be in play here but I think throwing them out without further information is a waste of time.