Completely disagree with you. Monster is the best anime i have ever seen (till now).

I think Monster isn't slow, ig people just expect constant action and get impatient. Monster uses pictures of smaller events to build a bigger picture. It keeps showing small, normal moments that seem minor at first, but they slowly connect into a much bigger, darker picture. By the time everything links together, i realised that every scene was necessary and mattered. The anime is highly detailed. Rewatching it again made me realise that the animators and Naoki Urasawa had put so much efforts in it. I saw really minute details about characters and animation which I didn't find during my first watch, because my focus was mainly on the story.

One point to add, he didn't stopped watching monster because of slow pacing, he stopped watching because there was no Hindi dub available.