Very awkward in portuguese too... no one says "tem perdido" (only those who have been wrongly influenced by english grammar construction). In fact, I think it is wrong to say it like that, you should say "Ele perde tempo com você" or "Ele está perdendo tempo com você". Anyway, the way Duo puts it ends up being more didatic and you probably be understood, but you will always sound like a foreigner (or like a bad telemarketing attendant or a bad secretary)... :/
But a bit awkward in English, and it would not mean the same thing. It would mean "While he has been with you, he has been getting behind in his work and risks missing his deadline." or someting analogous. It implies there is a certain amount of time he has for some endeavor and the clock is ticking.