Failure to upload attributes to large sets

We did the following:

Create a series of sets that had between 3 and 9000 participants in them.  Each participant had a couple of small columns of data.  We then tried first uploading the membership file defining what participants are in each set.  Then we tried to upload a file of attributes for each set (two small attributes).  For some of these large sets we got a 504 when trying to update the attributes.  Many minutes later on the attributes eventually showed up.

However, if you upload the entity file first (define the sets and its attributes) and _then_ upload the membership there is no problem at all.  The slowdown comes when trying to update attributes on existing set entities.