A 1080p export of a phone clip is often larger than it needs to be for a web page. Scaling to 720p and using a higher CRF with H.264 or VP9 removes data. The compressor here exposes width and a quality preset, then compares the byte counts after the run.
Trimming the timeline removes time you do not need. Removing the audio track helps only when the audio is a meaningful share of the file.
ffmpeg.wasm is slower than desktop FFmpeg and uses the device's memory. One conversion at a time is the safe default. If the tab becomes unresponsive, cancel and try a shorter or smaller source.