Pixel count dominates
Decoding and re-encoding scales with the number of pixels, so a 48 megapixel image takes roughly four times as long as a 12 megapixel one regardless of how many kilobytes each occupies on disk.
Source format matters more than target
Reading a JPG or PNG is fast. Developing a RAW file, rasterising a multi-page PDF or rendering an Office document through a layout engine are all substantially slower — those jobs are routed to a separate worker queue so they do not hold up quick conversions.
Upload speed is often the real cost
On a slow connection, transferring a 40 MB RAW file takes considerably longer than converting it. The progress bar covers both phases, which is why a big upload can look like a slow conversion.
Encoder effort
Compressing to WebP with the slowest, smallest setting (method 6) takes noticeably longer than the default. The compress tool uses it deliberately — the whole point there is the smallest possible file.