The pace of an anime adaptation is largely decided by the publication schedule of the manga beneath it. Serialisation determines how much story exists at any moment, and that supply governs everything.
Serialisation produces story at a fixed rate
Weekly manga appear in chapters of about twenty pages, and monthly titles in longer instalments, but either way the total available material grows on a schedule.
An anime episode typically consumes several chapters, which means a season can exhaust a year or more of publication in a matter of months.
That imbalance is the central constraint, since an adaptation running continuously will eventually catch up with a source that cannot accelerate.
Adaptations wait for a buffer
Producers generally do not commission an adaptation until enough chapters exist to fill a season with room to spare, which usually means several years of publication.
This is why successful manga are rarely adapted immediately, and why a series can be popular in print for a long time before appearing on screen.
The wait also serves as evidence of durability, since a title that has held its readership across several years is a safer proposition than one selling well in its first months.
Running out of source has known consequences
An adaptation that overtakes its source must either stop, slow down, or produce original material that diverges from the eventual published story.
Slowing down produces recap episodes and extended sequences that stretch a small amount of material, which audiences notice and generally dislike.
Original endings are the most damaging option, since the published work later contradicts them and the adaptation is left telling a story the author did not write.
The schedule pressures the original creator
Weekly serialisation is demanding, requiring a complete chapter every week with a small team, and the pace has well-documented effects on the health of the people producing it.
Hiatuses are common as a result, and each one reduces the buffer an adaptation depends on, which pushes any planned continuation further out.
Monthly serialisation reduces that pressure on the creator while making the supply problem worse, since a monthly title accumulates material far more slowly than an adaptation consumes it.
Adaptation choices are made against the buffer
When material is plentiful, a production can adapt closely and include material that develops character without advancing plot.
When it is scarce, the same production compresses, cutting subplots to reach a suitable stopping point with the chapters available.
Comparisons between an adaptation and its source therefore often reveal less about creative preference than about how much story existed when the season was planned.