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Many attachments or large attachments will have a significant effect on sync performance, mostly for the initial sync after an attachment is added.

In addition, the iPhone/iPod touch is a very constrained device. It's an impressive platform, but it does not have anywhere near the performance of even a slow modern desktop, and it is very limited in memory, to list a couple of constraints. Many attachments or large attachments can dramatically affect performance in accessing the corresponding items, and can effectively block access if the attachment size is enough to starve the system of memory.

I would say that it's fine to have the occasional attachment, but avoid having very many of them, and avoid attaching large documents.