Speed and Movement:
* climbed as slowly as a snail
* climbed as slowly as a glacier
* climbed as slowly as a sloth
* climbed as slowly as a clock hand
* climbed as slowly as molasses in January
* climbed as slowly as time itself
Effort and Difficulty:
* climbed as slowly as a drowning man
* climbed as slowly as a mountain climber in a blizzard
* climbed as slowly as a newborn learning to walk
* climbed as slowly as a ship in a dense fog
Other Options:
* climbed as slowly as a shadow creeps
* climbed as slowly as the sun sets
You can choose the simile that best fits the context and the feeling you want to convey.