I had this problem recently. But its also the first time I had been taking Modafinil and this other medication, both of which are known to potentially give you sexual disfunction. What was especially irritating is that in addition to it being much harder to cum, it also made it harder to main a very hard erection (lol pun).
I've usually always found that I am more likely to have problems cumming with a WG. In fact I remember having a problem my first time in about 2002, with some escort from BestEscort4U. At a certain point she says 'ok I give you 10 mins', or something. I had seen her for an hour and still not cum. I swear looking back it was still the most athletic sex I've had with a WG, maybe Im just really weak now.... anyway getting off topic ..... Im not certain if that was extra time or she was saying I had 10 mins left on the clock. At any rate maybe a year layer I got into a relationship that lasted a couple of years, I had premature ejaculation so bad with this girl at the start that it wasn't uncommon for me to cum pretty much straight away, or with just her rubbing herself on me.
From what I can determine from all my experiences, its really about the buildup and tension. With that girl above there was a lot of buildup, and she really turned me on. With a WG there is no build up, my brain has no time to get into it and get super turned on by them. I just have to immediately go into it because, you know, I just see pound coins and notes falling in front of my eyes as my time tickets down. They arent wrong when they say the brain is the most important erogenous zone. I thought that was more or less hippy crap for a long time.