MySQL Galera Cluster mass update delay
Galera replication is not synchronous, only what they call 'virtually synchronous'. It's synchronous only up to the point where the write set is to be applied.


  There might be a small period of time when a slave is out of sync.
  This happens because the master may apply events faster than the
  slave. And if you do read from the slave, you may read the data that
  has not changed yet.
  
  However, this behavior can be changed by setting the
  wsrep_causal_reads=ON variable. In this case, the read on the slave
  will wait until the event is applied (this will obviously increase the
  response time of the read). The gap between the slave and the master
  is the reason why this replication is called virtually synchronous
  replication, and not real synchronous replication.


Source: Multi-Master Replication (Percona XtraDB Cluster - this page also has a diagram which explains the problem).

Another approach is to define one of the nodes as the 'master', and then write only to that node. Critical reads should then also be directed at the 'master' node, whereas less-critical reads can go to any node.  

PS: Note that if you plan to write to all the nodes, you might find the following article useful: Galera’s big gotcha for MySQL users.

Edit:

Note that wsrep_causal_reads=ON has been deprecated by wsrep_sync_wait=1.