Speed up code generation in split_generated_files mode

[Millione]

Previously @missingdays supported split_generated_files mode to solve https://github.com/cloudwego/volo/issues/454, but users reported that the generation speed was 3-4 times slower than the previous mode, and the total number of lines of generated code increased by about 20%.

Such a slowdown for large projects is indeed unacceptable, we need to investigate to find out where the slowdown is and how to fix it.