Yes, slow is a Scrabble word!

Adverb

1. Without speed (`slow' is sometimes used informally for `slowly')

2. Of timepieces

Adjective

1. Not moving quickly; taking a comparatively long time

2. At a slow tempo

3. Slow to learn or understand; lacking intellectual acuity

4. (used of timepieces) indicating a time earlier than the correct time

5. So lacking in interest as to cause mental weariness

6. (of business) not active or brisk

Verb

1. Lose velocity; move more slowly

2. Become slow or slower

3. Cause to proceed more slowly