The effort may indeed affect the priority, and it also is potentially a factor in severity. If a customer knows that a high-severity issue will take a long time

The effort may indeed affect the priority, and it also is potentially a factor in severity. If a customer knows that a high-severity issue will take a long time to fix, they may decide on a mitigation for a particular delivery and lower the priority so that the fix comes in a patch or later delivery. The severity measure still counts in determining the overall quality of the release, though.