There is a rule in Competitive Tournament Magic that if a player was taking actions in a loop that has a non determinative outcome it is consider slow play as you are not advancing the board state. The big example is some combo where you can repeatedly activate an ability that mills the top card of your opponent’s deck but they have this card in their deck that when it is milled, it shuffles the entire discard pile back into the deck. Its not a great ruling because if you can do something between milling the shuffle card twice it is not slow play.
I have seen slow play that was clearly noticeable. Although, it might not have been on purpose. When a player thinks for a long time and then draws a card, thinks again, draws again, thinks again … This can be quite horrible, most of the time that happened, when I though that my opponent didn’t have a proper plan for the game and was just looking for answers without actually knowing what to look for. I haven’t called a judge for slow play yet. However, I try to get my opponent to understand that the clock is ticking and that we both have an interest in finishing the game. The worst experience for me was a player who achieved a narrow victory in a very long game and then expected of me, that I play the next game lightning fast, because he had the experience with the matchup and thought he could win quickly. I did call the judge for that. My opponent was quite vocal about my decision to call the judge and was actually complaining to him about me calling him when he arrived at the table. In the end the game was over quickly, because the player was disqualified for unsportsmanlike behavior. It wasn’t even a big tournament but I was so pissed, to see someone taking 57 minutes for the first game and then pretty much expected of me to lose quick enough, so he could get to 4 points …
Never saw him again after that event.
PS: A late reply I know, but I thought it was prudent to reply with a “slow post” considering the topic. 
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