When rates were higher, borrowers took on much less debt. A 1 or 2% increase in their mortgage rate represented a lesser percentage increase in the rate.on a much lower amount of debt. Today, someone with say an $800,000 mortgage outstanding facing an increase in mortgage rate from 3 to 5% faces a much larger % increase (66.6%) to their mortgage rate than someone in the past who was facing a mortgage rate increase from 10 to 12% (20% in this case) on say $150,000.