With a webcomic that only updates once a week, I admit it's hard. Even during ((SORTA-SPOILER FOR MY COMIC AHOOOYY)) the torture scene in my comic, I had one reader get annoyed at how long it was taking to get through it, when in reality it's not that long if you go through it in one sitting, it just felt slower because of the update rate. XP
It's hard to explain how to make it work. If you're building up a tense scene to one big climactic moment, drag it out for around one chapter rather than several. However, you can also build up tension for several chapters in different ways, like with Daniel, I tried to show many times throughout the comic that things are getting worse and worse with him when it comes to his sanity and his vampirism controlling his actions; that's building up tension in another way.
Other than that, really try not to worry about things being slow TOO much - go with your gut instincts even if you get a complaint or two about the pace. The worst thing you can do is jump right into the in-your-face stuff because you're worried it's not moving fast enough, it can erase all the tension entirely and suddenly your original plan for the story is lost.