The slowest part of a Pollen @racket[render] is compiling a source file. Because Pollen allows source files to be edited and previewed dynamically, these files get recompiled a lot. Therefore, Pollen stores copies of the exports of source files —namely, whatever is stored in @code[(format "~a" default-main-export)] and @code[(format "~a" default-meta-export)] —in a cache so they can be reused.
In each directory of your project, Pollen creates a subdirectory called @filepath{pollen-cache}. The files are stored on disk so they can be reused between sessions. If you delete files within a cache directory (or the whole thing), don't worry —everything will get regenerated. (However, you should not read or write to any @filepath{pollen-cache} directory, as the implementation details are subject to change.)
In each directory of your project, Pollen writes cache files into a subdirectory called @filepath{compiled}. The files are stored on disk so they can be reused between sessions. If you delete files within a cache directory (or the whole thing), don't worry —everything will get regenerated. (However, I don't recommend trying to read or write directly to any @filepath{compiled} directory, as the implementation details of the cache are subject to change.)
