cfg.Fast makes use of the J2V8 engine which increases the apps execution speed but does not have all the same facilities as the regular engine
That is why cfg.Fast is not the default
If your code does not work with cfg.Fast you will probably need to remove cfg.Fast from your code
Commenting it out will not work
If you need to test with and without cfg.Fast frequentl I suggest temporarily changing it to
cfg.xFast
More about cfg.Fast in the
Docs > Introduction > Coding features
or at
https://symdstools.github.io/Docs/docs/intro/04CodingFeatures.htm