It is likely the report is running just as slow but when you run it it appears to run quickly but the way it is rendering it hides that slowness.
You can test this by adding the 'page n of m' special field to the main report page footer (do not suppress that section or this field).
This will force the report to create all pages before it is rendered (similar to exporting it to a PDF).
I suggest this so you can try to better analyze if it is a PDF issue or a report run time issue.