How do I practice pacing on practice tests?

The value of practice tests, separate from other timed work, is putting all the elements of a real exam together in the way that most accurately simulates the actual test. You should never take a full timed practice exam with any intent other than to really replicate the entire situation and build your skill at that performance.

When you're taking a practice test, you want to have a plan ahead of time about splitting the section into multiple passes — how to make good, quick decisions about where you have a plan and how to implement it, and where you don't have a plan and should be saving something for later or skipping it entirely. You shouldn't be staring at the clock constantly and worrying about time, but you should be aware of how much time is passing on a per-question basis and whether what you're doing is reasonable or needs to be cut off. Just like an MC's job is to cut off a speaker who goes on too long and risks ruining the whole wedding with their hour-long speech, it's your job to cut off a single GRE problem that risks ruining your test.

One of the great features of practice exams is that they give you a record of how much time you spent per problem. Being able to look at that and think through what happened — where you spent too long, where you were efficient — is extremely valuable.