- `static let ` that cached values were replaced with `static var`.
- Some singletons were replaced with DI.

`static let` is very slow due to synchronization.

Every `static let` was removed, except for case with caching regular expression (it is faster to cache it in `static let`) and singletons (it is neccessary there)

Even `static let`'s for simple values like `Int` were removed. Compiler optimizes them to behave as with `static var`, however, for me it is better to not think about compiler optimizations, also, if there are only few `static let`, it is easier to track bad code like singletons or `static let` that lead to synchronization even with compiler optimizations.