Python Function Bundles Now Include Precompiled Bytecode

Vercel now compiles Python functions into bytecode at build time. According to benchmarks, cold start time for medium-sized functions dropped from 2.8 seconds to 1.3 seconds.

Previously, when Python imported modules without cached bytecode, it had to parse and compile the source code before execution. This step was a cause of slower execution start times for functions with large dependency trees.

Key features of the new approach include:

• Precompilation applied: Generated .pyc files from compiling both application code and dependencies are included in the function bundle.
• Execution efficiency: The interpreter skips the separate compilation process at startup.
• Automatic optimization: As much precompiled bytecode as possible is automatically added, based on the function bundle size.

No separate code changes are required, and the closer a function's size is to the limit, the less bytecode can be added, so the performance improvement may be somewhat smaller.