Performance issues in decdk renderFullSchema

[Chriscbr]

On my computers decdk takes over two minutes to build a simple hello-world stack, which seems a bit excessive.

Most of the effort appears to be spent in renderFullSchema(), which iterates through a lot of CPU-intensive loops while calling schemaForTypeReference(). Perhaps all this looping could be improved by building some lookup hash tables or other similar solutions?

It also seems that the rendered schema could be easily loaded from cdk.schema.json instead of generating it from scratch every time in the DeclarativeStack constructor.

Transferred from aws/aws-cdk#12349 (originally posted by @kennu).

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[Chriscbr]

As a post-mortem -- to measure performance issues, I timed how long different steps took while synthesizing a decdk template. Here's what I found:

• Loading the template from a file (8ms)
• Loading the type system (15sec)
• Parsing the template and creating the new DeclarativeStack (3sec)
• Synthesizing the CDK app into a template (38ms)

It turns out most of the time loading the type system was due to a validation step, which could be sped up (aws/jsii#3565). Now that step should take less than a second after jsii 1.60.0. There are still likely large performance improvements left, but this should be a good start!