The only thing I can say is that - don't worry about it. It's a pointless question since everyone is different. For example: I can take 3 days to write a 1.7k word prologue and yet take 3 hours for a 4k word chapter. In the end, it all boils down to the contents that are in it; how careful you are being; and how content you are with the quality of the writing.
My first story got the prologue, chapter 1, and chapter 2 on the same day. Which totaled to around 2.5k words. I really had no idea what I was doing, or what was a good length. I simply went for what I thought would be a good cut-off point. And now in the very same story chapter, 80 is 11k words.
Also as for speed, it can improve or decrease in time, it also depends on first draft writing standards and what you focus on improving. How well you have planned out character thoughts, for me dialogues are rather easy, for others they can feel often impossible. You can either focus on improving your first draft writing quality; which makes it turn out great but it is still the first draft. You can focus on speed and get a lot of typos, missing commas, among other things.
I personally just post chapters that are a husk of what they could be - first drafts. it all depends on writing styles, there are people that over-describe things, there are others that write it too plain. I am on the plain side, so a chapter can increase by 50% in word count through an edit or two. However, the best advice there is, in my opinion, is to turn it into a habit.
Write daily, don't give up, know what you are doing. Things like that - such as keeping notes of your story, simply actively writing. In the end; the most consistent one wins, not the fastest nor the highest quality. Consistency is probably your highest priority as a web-novelist. Although I must note that simply writing won't get you better at writing; it's all about your mindset.
Good luck o/
