は
Particle, pronounced わ in modern Japanese
1. topic marker particle
2. indicates contrast with another option (stated or unstated)
3. adds emphasis
そろそろ, ソロソロ
Adverb, Adverb taking the 'to' particle, Onomatopoeic or mimetic word
1. slowly, quietly, steadily, gradually, gingerly
2. soon, momentarily, before long, any time now
そろそろ寝る時間だぞ。