The usage of external scripts causes some slowdowns. (forking new processes, running interpreted scripts,...)
It can be interesting to keep an eye on this, though I don't expect major problems.

** History file size/performance
each new pageload -> fopen(history_file, "a"), fwrite one line, fclose.
I use utf8, so unless you use characters that are "special" (chinese etc) each character takes 1 byte.
So, assume each entry is about 80 chars, you visit 100 pages per day (?), and you wonder when your history file will be 50MB big:
(50 * 1000 * 1000 ) / ( 80 * 100 ) = 6250 days or 17 years.
There is code to run a benchmark in the 'extra' dir.  For results & interpretation, see http://dieter.plaetinck.be/poor_mans_dmenu_benchmark