One might refer to taking three points of a possible 15 as a "slump" without much fear of debate. Major League Soccer has ample examples of weaker performances: in 2015, Philadelphia has endured a stretch of 1 win in 11 games; New England has 1 win in its last 12; New York City FC took only 1 win out of its first 13 matches in the league. Still, the 0.6 ppg the Seattle Sounders have taken from their last five is the team's second such stretch. Only 4 teams in MLS history have extended such futility over the course of a full season ('99 Metrostars, '01 Mutiny, '05 Chivas USA, '13 DC). Seattle never took fewer than four points from a five-game stretch in last year's Shield-winning performance, and never took fewer than five in 2011. Since 2011, MLS teams have played 2,546 discrete sequences of five games within a season, and taken three points or fewer 383 times (roughly 15%).
The distribution of point totals in five-game windows is shown above for all MLS teams and for Seattle, specifically. Even with the two most recent examples, the Sounders only suffer such slumps 8% of the time. But then, Seattle is a perennial playoff team... is prone to hot and cold streaks when compared to MLS peers?
Seattle is pretty clearly more prone to weak stretches of play than the LA Galaxy - and offsets that trend to some degree with a remarkable tendency to take 10 points of 15 - but not significantly more streaky than Sporting Kansas City or the New York Red Bulls. This comparison is still somewhat complicated by differences in overall performance, with LA stronger and the others weaker - though we can see that the lack of a bell-shape curve to Seattle's distribution is unusual. We can only conclusively refer to Seattle as streaky when comparing the team to itself:
The first series in the bar chart is the now-familiar distribution of Seattle's actual results since 2011, while the series shows a simulation result: the average plus or minus 1 standard deviation when Seattle's 156 game results are randomly distributed 10 times. Over the last 5 years, the Sounders have been somewhat more prone to hot and cold spells than would be expected from random chance or the standards of strong MLS teams. Whether the trend is due to injury, fitness, or form - getting stuck in a rut isn't particularly fun from the perspective of a player or entertaining from the perspective of a fan.
Under the present circumstances, I could present some numbers demonstrating Seattle is missing uniquely important personnel, or that the sheer volume of important players is atypical of MLS play. Such excuses are ultimately satisfying to nobody. Gold Cup absences have long been expected, injuries happen (2014 notwithstanding), and poor performances in the absence of Osvaldo Alonso have been a problem for seven years. Slumps happen, but the team's susceptibility to such dips in form is itself a problem to be resolved.