General Slowness:
* Gradually
* Leisurely
* Deliberately
* Methodically
* Painfully
* At a snail's pace
* At a crawl
Slow Due to Difficulty:
* Laboriously
* Tediously
* Ploddingly
* With difficulty
* With effort
Slow Due to Lack of Interest:
* Reluctantly
* Indifferently
* Halfheartedly
* In a lackadaisical manner
Slow Due to Age or Weakness:
* Feeble
* Weakly
* Frail
* Decrepit
Slow Due to Time:
* Gradually
* Incrementally
* Step by step
* Little by little
Figurative:
* At a turtle's pace
* Inch by inch
* At a snail's pace
* At a glacial pace
The best synonym for "slowly" will depend on the specific context and the nuance you want to convey.