If your vehicle is showing noticeable loss of power when accelerating or driving uphill, here are the most likely causes and the parts you'll need to fix each one.
Under heavy load like driving uphill, cylinder pressures increase, making it harder for a spark to jump the gap. Worn ignition components will fail to fire properly, causing misfires, hesitation, and a noticeable loss of power.
Accelerating and climbing hills requires a higher volume of fuel. A restricted fuel filter or a failing fuel pump will starve the engine of the fuel it needs under load, causing it to bog down.
The MAF sensor measures the amount of air entering the engine to calculate the correct fuel mixture. If it is dirty, it will under-report airflow during acceleration, causing the engine to run lean and lose power.
A restricted catalytic converter acts like a potato in the tailpipe. At idle it might seem fine, but under heavy acceleration, the exhaust gases cannot escape fast enough, choking the engine and severely limiting power.
If the engine revs up but the vehicle doesn't accelerate proportionally, the power is not reaching the wheels. This is typically caused by a worn clutch disc in manual cars or degraded fluid/internal wear in automatics.
Unmetered air entering the intake, or pressurized air escaping in turbocharged vehicles, ruins the air/fuel ratio. This is often caused by loose clamps or disconnected hoses after recent engine work.