Can I slack on the job?
Why would you slack on the job that they are paying you to do?
I've been given a huge workload and I've decided to slack on it to go get a coffee
That is immoral of you to do that on the job. It's good to always do your best at tasks you undertake. They are paying you to do a good job and you shouldn't slack off because of your workload.
That's my answer. Fuck what the manager says.
You aren't doing a good job for the manager, but rather yourself. Imagine what will happen if you're written up for slacking on the job, or if your coworkers notice? You could lose a lot more than just a few hours of extra annoying work if you're thought of as a bad employee and so you should always give each job your best.