Janino currently uses repeated calls to the String.concat() method to implement the concatenation of strings using the + operator. While this is certainly optimal in the case of two operands, larger numbers of operands result in degraded performance (due to the resulting quadratic complexity). Though Janino performs a 'constant folding' optimization that resolves this in many/most instances, the performance of non-constant concatenation is still a problem. A test case can be dowloaded from: http://www.tomgibara.com/janino-evaluation A preliminary benchmark comparing the performance of concat() to StringBuffers and StringBuilders in Java 1.5 and Java 6 might be useful and is available at: http://www.tomgibara.com/janino-evaluation/string-concatenation-benchmark Note this behaviour has been observed in Janino versions 2.5.1 - 2.5.3 inclusive, but is probably present in all versions.