diff --git a/src/repr/chains.rs b/src/repr/chains.rs
index 46ab7ec..6566853 100644
--- a/src/repr/chains.rs
+++ b/src/repr/chains.rs
@@ -84,6 +84,7 @@ pub fn gen_adj_lookup() -> PosMap<ChainCollection> {
                                 .map(Iterator::collect),
                         )
                         .chain(diag_raw(i_chain, j_chain).map(Iterator::collect))
+                        .filter(|x: &Vec<(u8, u8)>| !x.is_empty()) // PERF! filter out empty chains +~10-12% perf boost
                         .map(|x: Vec<(u8, u8)>| x.into_iter().map(|x| x.into()).collect()),
                 );