The counterargument should be obvious -- peer reviewers are often unpaid experts who review pending articles in whatever spare time they happen to have, between delivering lectures, research, and applying for grants. To speed up the process woulds be to increase its cost, possibly very much, or to degrade the peer review process.

The defects of another alternative should also be obvious -- paid peer reviewers. If this change took place, people would line up for positions they aren't qualified for on the simple ground that they can get paid to appear to be what they aren't. Also, for many cutting-edge fields, there simply aren't more than a handful of qualified reviewers at any price.