Jekyll not fast enough? Why not build a compatible faster clone /
alternative from scratch / zero (in ruby)?  Sounds crazy?
Allan Odgaard (of TextMate fame) has just done it and with about 3
000 lines of ruby code offers even more features than jekyll itself
e.g. built-in tags and categories for collections, lazy evaluation
and parallized builds, better defaults, and much more. Find out more
at the Glim website compiler source project repo 
Happy publishing w/ ruby. Cheers. Prost.
PS: By the way - did you know? Slideshow (S9)  is another
jekyll-compatible clone that lets you build presentations / talk
slides in markdown (kramdown, really) and jekyll themes  e.g.
reveal.js, shower.js, bespoke.js, s6, etc.