This is quite surprising, I would have expected the performance to be somewhere between a hand-written parser and a regex parser but it probably depends on the grammar and the lookahead it has to perform. Have you tried profiling the parser to see if there's not some unexpected bottleneck?

I did a lot of profiling on PackCC and it turns out it is not very efficient with memory allocations (see universal-ctags/ctags#2866 (comment)), the backtracking is constantly allocating and deallocating small chunks of memory. So, more rules leads to more backtracking, which leads to more allocations, which slows it down. I have even proposed couple possible optimizations, but I did not have from enough manpower to actually implement it (sadly, I am not very proficient in C).

I might look into the grammar optimization in future, so the parser might become somewhat faster (especially, if it slows Geany down, that would annoy me a lot). Probably could beat the RegExp based ones (for comparable grammars). According to creator of ctags, the peg parser is about 15-30 times slower than handwritten parser (citation from universal-ctags/ctags#2866 (comment)). Even with grammar optimizations, it would still probably be about an order of magnitude slower than let's say current C parser.

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