I don't know if it's a problem with IE specifically, I'll have to go test now, but we've had some issues with reports running ridiculously slow once deployed, when they run fine in BIDS. To fix that, the common workaround seems to be (after googly-moogling ;-))to pass the parameters into your sp, then immediately store into local variables, and use the local variables to retrieve the data.

Not sure why that works, sounds sorta like parameter sniffing problems, but you're still using a variable, so no idea how that would fix the problem.

hope that helps,

Jon