。
Conversion takes quite a lot of time.
Literal
Conversion [for-には] considerable [of-の] time [subject-が] takes.
Tech/computing register. 変換 'conversion' is the standard term in computing for IME conversion (kana to kanji), format conversion, etc. かなりの 'considerable, quite a lot of' with の attached turns the adverbial かなり into an attributive modifier. 時間がかかる 'time is taken / it takes time' is a fixed expression where the logical subject (time) takes が, not を. A terse technical observation.