when you spam a simple /cast button the server queues your skill as to avoid being influenced by latency.

If you ever run into latency problems and get like 1kms or something, you would notice you can still spamheal or spamdps one skill at a decent rate.

However, castsequences do not queue on the server, so the next cast after the last doesn't get sent until the first is launched. And therefor adds the time of your latency inbetween every cast.