also: といってよいほど
expression
so much that one could even say; practically; almost
Used to emphasize the degree of something, often followed by a strong statement. Indicates that the situation is so extreme that one could even say the following phrase.
彼は毎日のように遅刻する。と言っていいほど、時間にルーズだ。
He is late almost every day—so much so that you could even say he is careless about time.
この料理は、と言っていいほど辛くない。
This dish is practically not spicy at all.
ほとんど means 'almost' or 'nearly' in a straightforward way. と言っていいほど adds a rhetorical emphasis, implying that the statement is so true one could even say it.
と言っても過言ではない is a more formal and stronger expression meaning 'it is no exaggeration to say'. と言っていいほど is slightly less formal and more conversational.
A fixed expression composed of the quotative particle と, the te-form of 言う (to say), いい (good/okay), and ほど (extent). Literally 'to the extent that it is okay to say'.