I am afraid this benchmark does not really reflect real world scenarios. ReScript creates a lot of intermediary files for every .res file. There are ideas around this like putting more of that into a in-memory database, as most codebases would fit entirely into the RAM of modern machines, but at least a rescript build performs much better than a rescript-legacy build.
I am not sure how TypeScript handles it internally, but I think it creates fewer files and thus there is less I/O.
If there were more type definitions, possibly interdependent ones, this benchmark would be more interesting.