What would be reason behid WaitType PAGEIOLATCH_SH?

I am executing a stord proc that basically truncates & reload the table x.During reload of data , It joins table a , table b that contains 100 millions & 2 millions records resp.During the execution of stored proc i found in activity monitor that WaitType PAGEIOLATCH_SH is slowing down my sp. what would be the reason behind this waittype "PAGEIOLATCH_SH". And how to avoid this waittype.

Thanks in开发者_JAVA技巧 Advance.

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