Here's the thing: a client doesn't really know (or care) whether you're slow or not compared to other developers. All they really care about is whether you're delivering the positive benefits they hoped you would achieve when they hired you, for a price that justifies their investment, in the time frame they expected.
While you will get faster in time, here's what you should focus on if you want to succeed (hint: it has nothing to do with your speed):
- Set the client's expectations appropriately: If you have a project to work on, don't just say "great, I'll start ASAP and let you know when I'm done." If you think it will take about 2 weeks, pad that number a bit and tell them "great, I'll start ASAP - it should be done in about 3 weeks." Then when you finish in 2 weeks, you look really fast. And if it takes longer than expected, you still deliver on time.
- Send consistent updates: Give your client regular, unsolicited updates on their project. For example, send them a quick status report every Friday with what you accomplished and how it ties in to their business goals. If you hit a roadblock, let them know early that their project will be delayed. It's much better to tell the client about delays early in the project than tell yourself "maybe I can make up the lost time later in the project." It rarely happens (trust me) and then you break the client's trust when you miss your deadline. If you proactively communicate timeline changes, you actually build your client's trust, which seems counter-intuitive. But speaking from experience, this is the way to go.
If you focus on providing the client great service and keeping your promises to them, it doesn't matter whether you're 500% slower than everyone else. The client most likely won't know the difference, and even if they do, they may just assume you're juggling a lot of other projects. And if the project delivers the positive benefits they were hoping for, you will probably get recommended over many other "faster" developers. In the end, clients don't hire you to write code fast, they hire you to solve problems for them and deliver those results in a professional way.