slow reply; late response
Internet slang for replying to a message, email, or post after a noticeably long delay. Often used self-deprecatingly or to tease someone.
See also: レス
Sorry for the slow reply.
彼からの返事はいつも亀レスだ。
His replies are always late.
Compound of 亀 (turtle, implying slowness) and レス (abbreviation of 'response'). Internet slang.