A sag is rarely caused by a slow scene. It is caused by a stretch of pages where the question the audience is holding stops changing. Momentum comes from a live question with a moving answer, not from action, volume, or incident.
Check three things in order. First, escalation: does each sequence cost the protagonist more than the last? If your setbacks are interchangeable — they could be reshuffled with no damage — you have repetition, not escalation. Second, the midpoint: something must reframe the goal, reveal information that changes the risk, or convert the protagonist from reacting to pursuing. Third, the opposition: a passive or absent antagonist produces a sag every single time, because pressure has to come from somewhere.
Subplots are load-bearing here. A subplot that only supplies texture will not hold weight — it needs to pressure the main want, ideally by making the protagonist choose between the two.
Cutting is a legitimate structural tool, not surrender. Most sagging middles are twelve pages of a good idea explored three times.