Generally speaking, underperforming for two halves (so, a year) is very likely to result in a dismissal. Even earlier, if it's particularly bad. Senior Engineer (level 5) is considered terminal, i.e. you won't get fired for not growing, but at that point expectations are high, so coasting is not really an option for most people. Lower levels have a ticking clock, with the expectation of reaching senior by a certain time at the latest. Not doing so will definitely result in a dismissal.
TL;DR, it is a high-performance culture and ignoring it is hardly realistic. People do get let go. Folks usually leave on their own before it gets to this point.