g:uctags_use_readtags

Default: 1
As mentioned on the official Universal-Ctags Docs about readtags:

Currently this feature is only available on platforms where fmemopen is available as part of libc. Filtering in readtags is an experimental feature.

Prior to the implementation of this feature into Vim-UCTags, the way tags were searched was by manually parsing through the entire tags file. From a coding point of view, this was very tedious and on occasion was a bottleneck, especially when the tags file had an immense amount of lines. With that said, the performance increase is most likely marginal at best with hope in being greater in the future.

Similar to g:uctags_use_perl, there are Vim and Perl variant procedures that are utilized when not using readtags.