Usually, when I'm previewing a timeline with heavy effects already added, in order to assess motion-related aspects (which requires full fps playback) - I either lower the preview quality temporarily, or press the "bypass effects" icon above the preview window. Usually, the latter method works a treat, and I'm getting full 25 fps...
However, with some plug-ins in the timeline, I've just noticed this method might not be enough. When both Mercalli V2 and Neat Video are present in the FX chain, in order to get the full fps preview I need to actually "disable all" (in the events, or track - wherever they are placed).
Strange, huh?
Piotr