Swiftを用いたCoding Interviewに向けて、Time Complexityについて整理します。
Arrayでよく利用するmethodを中心に一覧にしましたので、ご参考になればと思います。

公式のドキュメントに丁寧に Time Complexityの記載はありますので、
詳しくは公式のドキュメントを参照してください。
https://developer.apple.com/documentation/swift/array

チートシート

Name	Complexity	Note
popLast()	O(1)	Removes and returns the last element of the collection.
removeFirst()	O(n)	Removes and returns the first element of the collection.
removeLast()	O(1)	Removes and returns the last element of the collection.
dropFirst()	O(1), O(k)	Returns a subsequence containing all but the given number of initial elements.
dropLast()	O(1), O(n)	Returns a subsequence containing all but the specified number of final elements.
drop(while:)	O(n)	Returns a subsequence by skipping elements while `predicate` returns `true` and returning the remaining elements.
prefix()	O(1), O(k)	Returns a subsequence, up to the specified maximum length, containing the initial elements of the collection.
prefix(while:)	O(n)	Returns a subsequence containing the initial elements until `predicate` returns `false` and skipping the remaining elements.
prefix(upto:)	O(1)	Returns a subsequence from the start of the collection up to, but not including, the specified position.
prefix(through:)	O(1)	Returns a subsequence from the start of the collection through the specified position.
suffix()	O(1), O(n)	Returns a subsequence, up to the given maximum length, containing the final elements of the collection.
suffix(from:)	O(1)	Returns a subsequence from the specified position to the end of the collection.

Name	Complexity	Note
map()	O(n)	Returns an array containing the results of mapping the given closure over the sequence's elements.
flatMap()	O(m + n)	Returns an array containing the concatenated results of calling the given transformation with each element of this sequence.
compactMap()	O(m + n)	Returns an array containing the non-`nil` results of calling the given transformation with each element of this sequence.
filter()	O(n)	Returns an array containing, in order, the elements of the sequence that satisfy the given predicate.
reduce()	O(n)	Returns the result of combining the elements of the sequence using the given closure.
first()	O(n)	Returns the first element of the sequence that satisfies the given predicate.
enumerated()	O(1)	Returns a sequence of pairs (n, x), where n represents a consecutive integer starting at zero and x represents an element of the sequence.
min()	O(n)	Returns the minimum element in the sequence, using the given predicate as the comparison between elements.
max()	O(n)	Returns the maximum element in the sequence, using the given predicate as the comparison between elements.
contains()	O(n)	Returns a Boolean value indicating whether the sequence contains an element that satisfies the given predicate.
reverse()	O(n)	Reverses the elements of the collection in place.
reversed()	O(1)	Returns a view presenting the elements of the collection in reverse order.
sort()	O(n log n)	Sorts the collection in place, using the given predicate as the comparison between elements.
sorted()	O(n log n)	Returns the elements of the sequence, sorted using the given predicate as the comparison between elements.
swapAt()	O(1)	Exchanges the values at the specified indices of the collection.
split()	O(n)	Returns the longest possible subsequences of the collection, in order, that don't contain elements satisfying the given predicate.
shuffled()	O(n)	Returns the elements of the sequence, shuffled.

注釈

O(1), O(k) : O(1) if the collection conforms to RandomAccessCollection otherwise, O(k), where k is the number of elements to drop from the beginning of the collection.
O(1), O(n) : O(1) if the collection conforms to RandomAccessCollection; otherwise, O(n), where n is the length of the collection.
O(n) : n is the length of the collection.
O(m + n) : n is the length of this sequence and m is the length of the result.
O(n log n) : n is the length of the collection.

個人的な見解

removeFirst()は Collectionタイプでは O(1)であるが、Arrayタイプの時はO(n)となる。これはArrayにはindexが付与されており、先頭のElementとremoveした場合残りの全てのElementのIndexをdecrementする必要があるため。
reversed()は Collectionタイプでは O(n)であるが、Arrayタイプの時はO(1)となる。これはArrayの時は ReversedCollectionインスタンスが、基になるコレクションをラップし、その要素へのアクセスを逆の順序で返すため。