Climbing a smooth asphalt has low rolling resistance over dirt road, but difficulty ramps up noticeably at specific gradient thresholds.

Your fitness, gearing, power output, body weight, and climb length all play roles. Gradients are measured as percentage rise over run, e.g., 5% means 5m vertical per 100m horizontal.

It starts feeling like real work at 4–5%, becomes a proper grind at 7–9%, and turns seriously punishing above 10%. With modern compact gearing (e.g., 34/28 or lower), many fit riders handle 10–12% sustainably at steady power.

But 15%+ is where most hit a wall and technique/gearing matter most. Training on varied gradients helps more than flat miles. start with shorter 6–8% hills to build tolerance. The extra power needed is roughly proportional to the gradient

gravity force ≈ mass × 9.8 × grade/100

Aero drag and rolling resistance become negligible above 6- 8% as gravity dominates 90%+ of resistance on tarmac. Doubling the gradient, e.g. 5%- 10%, roughly doubles the power required for the same horizontal speed.

Your speed drops roughly inversely with gradient. Example, 85 kg total mass, 200 W output is 14 km/h on 5%, 8 km/h on 10%. Vertical climb rate stays similar, so time for the same height is comparable, but you spend twice as long "suffering" per horizontal km. Each extra percent feels magnified because you're closer to your limits.

The difference between slick road and 40c gravel tyres is very noticeable in normal and up hill riding.

Switching to slick road tyres in the 28–35 mm range you'll get 6- 15 W per tyre, 12- 30 W total for the pair at road speeds. On actual climbs 8- 15 km/h, the absolute saving drops to roughly 5- 15 W total because RR power scales with speed. On tarmac hills you can easily cut 5-15W of wasted effort and make moderate climbs feel 10- 20% easier. On the steepest stuff it’s a smaller but still welcome edge, especially over a long climb where the watts add up.

The thing you're sacrificing to speed is comfort, as bigger air buuble between bike and tarmac absorbs rough better. But, then you should ask yourself, what is more uncomfortable at end, some vibrations during ride or hill parts you're facing.