The current version of brew has a flaw where the installer can't install isolated dependency trees in a sterile manner. If you have packages A, B, C, and D that all have updates, and assuming A,B,C depend on each other and come out to a total of say 1MB, and D is 1000MB, brew works in a MapReduce manner where it will attempt to finish downloading everything in parallel (even though the real bottleneck is D) before doing any installation.

Since the first 3 has no dependency on D, a better way would be to install them in parallel while D is still downloading.