Focusing on the slowness:
* Plodding: This implies a slow, steady, and sometimes laborious pace.
* Lumbering: Suggests a heavy, awkward movement.
* Sluggish: Implies a lack of energy or speed.
Focusing on the persistence:
* Tenacious: This emphasizes their determination and unwillingness to give up.
* Relentless: Highlights their unwavering effort, even in the face of difficulty.
* Unflagging: Indicates their undiminished energy and enthusiasm.
Combining both:
* Persevering: This captures both the slowness and the determination.
* Tireless: This suggests they are not easily worn down, even with their slow pace.
The best adjective will depend on the specific context and the nuance you want to convey.