Not for string search, but I got fed up with the extremely long time it takes less to precalculate line counts on large files when I was working with log files of a size in the order of GBs. The result is here: https://github.com/nhaehnle/less

Since it was only a small hack to scratch the itch I was having at the time, I never really completed that project. For example, backwards line counting is not sped up, which can sometimes be noticeable.

If you feel like working on less-speedup issues, feel free to drop me a line.