Why this is fast
Two reasons. First, the encode itself is native ffmpeg, not WebAssembly — that is roughly 4–8x the throughput of in-browser ffmpeg on the same hardware, because the server runs real SIMD, threading, and direct disk I/O instead of the WASM shim. Second, the round-trip is short. Uploads go to the nearest Backblaze B2 point of presence via a presigned URL, so a viewer in Sydney is not pushing bytes through a single US-east origin. The finished MP4 comes back through Bunny CDN from the same edge. The result: a one-minute screen recording is usually done in seconds, including upload.
What this tool does, exactly
We change each frame's presentation timestamp with setpts=(1/speed)*PTS and run the audio through a chained atempo filter (atempo only accepts 0.5–2.0 in a single instance, so 4x speed-up becomes atempo=2.0,atempo=2.0). Video is re-encoded with libx264 at CRF 22 (visually transparent), audio is AAC at 160 kbps, and the output MP4 carries +faststart so it plays the moment it starts streaming. Frames are re-timed, not duplicated or dropped for moderate ratios (0.5x–2x); extreme ratios drop or hold frames but a 30 fps source still reads as smooth.
Why pitch correction matters
The naive way to speed up a video is to play frames faster, which also pitches the audio up — that is the chipmunk effect. Naive slow-mo has the opposite problem: voices drop into a Darth-Vader register. atempo decouples time from pitch using a phase-vocoder, so voices stay at the same frequency and words just arrive sooner or later. The result sounds like a podcast app running at 1.5x — not a cartoon. The same setting works for 0.25x slow study playback all the way up to 4x condensed meeting recaps, with no audible artifacts on speech.
Sister tools
Speed-changed video too big now? Video compressor shrinks it without a visible quality drop. Need a true slow-only flow? Slow motion video maker is the focused 0.25x/0.5x/0.75x preset. Want to cut the speed-changed clip down to one section? Video cutter handles that. Need a frame as a still for a thumbnail? Video thumbnail maker. Going silent for a time-lapse? Mute video strips the audio losslessly. If you want viewers to control speed instead of baking it in, record on Clipy — the hosted player exposes 0.5x/1x/1.25x/1.5x/2x without re-exporting.