I work for a commercial synthetic DNA producer in R&D. At the moment writing DNA is the slow part. There has been an almost Moore's law like reduction in price/increase in speed for sequencing, but so far synthesis has lagged. The state-of-the-art is phosphoramidite synthesis which has been around since the 80's. We have optimized and streamlined the process, but a single base addition (writing one 'bit') is still in the range of 100's of seconds. Also the length of DNA produced by phosphoramidite synth is limited to a couple hundred bases in most cases (although it's possible to extend that out to hundreds of bases at increased cost/reduced efficiency). So long DNA has to be stitched together from shorter strands. The other issue with synthetic DNA is quality so methods like microarrays are not necessarily the best (quality cannot be objectively measured on a per strand basis).

There are some interesting developments out there, like using enzymatic methods to synthesize long strands, but a lot of that technology is still in it's infancy.

I would be happy to discuss the field if anyone is working in it (or has interest)