It is unclear to me why that is happening. Is the video playing (slowly) in Blue Iris or is it stuck/frozen?
Try putting 15 in the axis's "Frame rate (fps)" field. Blue Iris has a "Max. rate" field which you should ignore because that is for things like USB webcams where Blue Iris actually could control the frame rate.
Put 15 in the "GOP length" field.
Do that for both the main stream (high res) and the sub stream (low res).
Then for good measure, in Blue Iris, right click that camera instance and "Restart camera".
If that doesn't solve it, try opening those RTSP streams in a 3rd-party video player like VLC to verify that they are streaming at the appropriate frame rate.
Try putting 15 in the axis's "Frame rate (fps)" field. Blue Iris has a "Max. rate" field which you should ignore because that is for things like USB webcams where Blue Iris actually could control the frame rate.
Put 15 in the "GOP length" field.
Do that for both the main stream (high res) and the sub stream (low res).
Then for good measure, in Blue Iris, right click that camera instance and "Restart camera".
If that doesn't solve it, try opening those RTSP streams in a 3rd-party video player like VLC to verify that they are streaming at the appropriate frame rate.