improve the perf

[jianyexi]

For some big swagger, it seems too slow, the problem is that the tool does the below thing during its run
1 use autorest(v2) to transform the swagger and get a resolved swaggger and source mapping file
2 transform the swagger allOf and path level parameter to make it fit the modeling which is defined in C#

when we consider improving the perf, we can focus on above points