After landing bug 914532 exporting contacts to the SD-card happens in batches of 2MiB; this causes the output to be split into multiple files each 2MiB in size if the total length of the exported vCards is more than 2MiB. Once bug 859696 lands we should change this so that all contacts are incrementally written to a single file. Attachment 832262 [details] [diff] already did that and can be used as a reference. In addition to this we currently process the contacts in batches only in the output phase but we still load all the contacts in one go. This also poses a scalability problem when trying to output large number of contacts and should be replaced via incremental loading. Bug 914532 comment 35 contains an example of how to do this.
Now that bug 855952 has landed we could use that to implement the functionality here instead of waiting for the more generic solution in bug 859696. I'm changing the dependencies to reflect that.