When sending a stash across the network, the data transfer can be significantly slower than the underlying network. This seems to be due to some slow on-the-fly compression happening during the transfer. Using the following script, the Unstash phase takes 1-2 seconds for 10MB of /dev/zero, and 49 seconds for 10MB of /dev/urandom. The Stash phase consistently takes 1-2 seconds. This makes me think there's a compression step, since /dev/zero should compress much faster than /dev/urandom.

node {
    deleteDir()
    stage "Stash"
    sh "dd if=/dev/zero of=data bs=1M count=10"
    stash name: 'build_outputs', includes: 'data'
    sh "date"
    node('lab') {
        deleteDir()
        stage "Unstash"
        unstash 'build_outputs'
        sh "ls -al"
    }
}

This looks similar to JENKINS-36914, in particular this comment:
https://issues.jenkins-ci.org/browse/JENKINS-36914?focusedCommentId=268472&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-268472

I'm not calling this a dupe of that because the original text refers to stash only, and this bug seems isolated to unstash. Also this is on x86, not ARM.