A Systematic Study of Time Limit Exceeded Errors in Online Programming Assignments

Online programming platforms such as Codeforces and LeetCode attract millions of users seeking to learn to program or refine their skills for industry interviews. A major challenge for these users is the Time Limit Exceeded (TLE) error, triggered when a program exceeds the execution time bound. Although designed as a performance safeguard, TLE errors are difficult to resolve: error messages provide no diagnostic insight, platform support is minimal, and existing debugging tools offer little help. As a result, many users abandon their submissions after repeated TLE failures. This paper presents the first large-scale empirical study of TLE errors in online programming. We manually analyzed 1000 Codeforces submissions with TLE errors, classified their root causes, and traced how users attempted to fix them. Our analysis shows that TLE errors often arise not only from inefficient algorithms but also from infinite loops, improper data structure use, and inefficient I/O, challenging the conventional view that TLEs are purely performance issues. Guided by these findings, we introduce Nettle, the first automated repair tool specifically designed for TLE errors, and Nettle-Eval, the first framework for evaluating TLE repairs. Integrating LLMs with targeted automated feedback generated by the compiler and test cases, Nettle produces small, correct code edits that eliminate TLEs while preserving functionality. Evaluated on the same 1000 real-world cases, Nettle achieves a 98.5% fix rate, far exceeding the strongest LLM baseline, and all of its repairs pass both Nettle-Eval and the platform's official checker, confirming the reliability of our framework.