This is the 1024th most frequent Russian word.
заметно
Noticeably.
In this sentence, "заметно" is used as an adverb meaning "noticeably." It describes the degree to which "colder" is sensed, emphasizing the detectable change in the weather.
Сегодня на улице заметно холоднее.
Today it is noticeably colder outside.
Here, "заметно" indicates that the appreciation was perceivably clear to others, implying that his efforts were recognized in a notable way.
Его старания были заметно оценены коллегами.
His efforts were notably appreciated by his colleagues.
In this example, "заметно" modifies "slowed" to highlight the degree to which the slowing of progress could be observed.