Everything BasarahBasarah said is true. Especially the last part, lol.
But seriously, there's a lot of things that need to be taken into consideration. For starters, since you didn't mention anything about pay, I'm going to assume that the artist is either working for free, or for very cheap, or maybe for a percentage of profits if there are/will ever be any. If this is the case, you have to understand that no matter how much someone loves to draw, they still have to make a living. He may be busy working another job that pays what he needs and hasn't had time to work on yours. That shouldn't justify extremely slow work, but it is a factor in the equation.
Have you ever gone over a schedule with him? You need to say, "Ok, we update on X day, which means you have to get X page(s) to us no later than two weeks prior to that day." Give him REAL numbers. Set dates. You'd be surprised to know that this actually helps a lot. I do illustrating for a living, and when a client tells me they want something done as soon as possible, this drives me nuts. It tells ME that they aren't really that prepared otherwise they would have asked for said illustration when there was a good amount of time to complete it. Granted your situation is a little different, but the point I'm trying to make is that giving exact deadlines, down to the exact day of the month, is very helpful. If he can't meet deadlines, then that's a big problem and you shouldn't have to keep him on. Deadlines are deadlines and shouldn't have to be bent constantly. Once in a while because of extenuating circumstances...ok. But constantly missing deadlines means that the illustrator is not doing his job properly. Period.
Also understand (and perhaps you already do) that drawing a quality comic page is not a snap-your-fingers-and-it's-done type of project. Most illustrators take more than an entire day to just pencil and ink a page. A lot of pro comic illustrators take an entire day just to pencil a page. Sure it depends on the style and the technique and the project. But producing a quality comic is NOT fast. I significantly "dumb down" my art for my webcomic, And To Be Loved, and I still feel like it takes me forever to complete a page and that I don't have the time for it.
Another word of advice I can give is to pick up a copy of Dave Sim's "Cerebus Guide to Self Publishing." Try to get the newest edition if you can (2010, I think). I talk about this book a lot, and anyone who has ever read it knows why. It has some of the best advice for aspiring creators you can possibly find. It's more geared towards the printed comics industry, but the newest edition does have a chapter on webcomics. But either way, there's a lot of helpful information in there about collaborating and he sheds light on the intense job of an illustrator. What I love most about it is that he doesn't sugar-coat anything. He tells it like it is. He talks about how hard the work is and how none of us are special. As a writer/illustrator, he can fairly say how much more work is involved with illustrating as opposed to writing. Not to say that writing isn't tough, but illustrating is, typically, much tougher and takes more time. Yes, there are exceptions to this blah blah blah.....
But yeah, as BasarahBasarah suggested, just talk to the artist. See what his deal is, and try to work something out. DON'T go into it with expectations of ANY KIND. If he tells you something you don't expect to hear, don't be surprised. Whatever that may be. But at least then you'll know what's going on and you can move forward from there. If this is someone you want to work with, try to negotiate and be willing to meet half way. If you can't work something out, tell him thanks for his time but you'll need to find someone that you can rely on more. Yes, you may need to draw it yourself. But that doesn't mean it'll be bad. You'll just need to find a style that works. Some of the most successful webomics out there are drawn (and written)terribly, imo, so that should tell us that anything is possible.