For large batches of dusty old slides, the best solution is usually not a software filter after scanning, but re-scanning with hardware dust removal.

If you still have the original slides, clean them first with canned air, then use a slide scanner that supports Digital ICE (or a similar infrared dust-removal system). It uses an infrared pass to detect dust and scratches and automatically removes much of it with far less detail loss than generic image filters. Renting a good film scanner or using a scanning service can be more effective than trying to batch-fix existing scans.

Software-only filters can help somewhat, but they’re a compromise. Dust/scratch removal generally requires selective masking and filters such as median filtering; simple blur filters like Gaussian blur will soften detail too much. Fully automatic batch processing on already-scanned images is likely to leave artifacts or reduce sharpness, especially on high-resolution scans.

So: if quality matters, re-scan with infrared dust removal. If that’s not possible, software cleanup may be acceptable for smaller display sizes or modest prints, but it won’t be as clean or as detailed.