If your bike is anything like mine then after a couple of bike rides in the rain, the rims and brake pads will be fairly caked in road muck. This greatly reduces braking performance and speeds up the wear on the rims and the pads.
To get them clean, the best method is to use surgical spirit (rubbing alcohol) and a clean rag. Ideally, you’ll also use a small spray bottle to spray it on to the rim surface. Surgical spirit is sold in pharmacies.
Spray the cleaner on to both sides of the rim and gently apply your brakes. This will start to work it in to the brake pads and the rims. As the cruds starts to loosen and run down the rim, use a clean cloth to wipe it away.
Alternatively, if the wheel is on the bike then simply dab some surgical spirit on to a cloth and wipe your brake pads and rims.
This is also a good time to check your brake pads to see if that are worn. Look for an indicator line or if that is lacking check that there are still grooves in the pads.
On your next bike ride you should notice an improvement to braking performance.