In letzter Zeit is not freely interchangeable with other words meaning “lately, recently”, such as kürzlich, neulich, letztens, all of which refer to a particular time or event in the recent past, as in: Ich habe sie kürzlich besucht. (“I visited her lately.”) — Compare to this: Ich habe sie in letzter Zeit besucht, which will typically mean a number of visits throughout the recent past.