I was wondering whether there is anything I can do to improve the speed of reviews.
You can request an expedited review, but you should only do that when there's a legitimate reason that you need to get your app out there ASAP, such as a glaring security bug. Otherwise, just allow time for the review process in your schedule.
being able to adapt the sales channel faster than the 9-10 days delay cause by the app review times is crucial.
The best way to deal with the issue is to design your app so that you don't need to update the app itself in order to adjust your sales channel. Anticipate the ways you might want to change the app and build all the required behaviors into your app. Then use data files to tell the app how to behave.
A simple example: instead of hard coding your product and price list into the app, put that information in a data file. Have the app download the latest file when it launches for the first time, and check for updates periodically.
A more complex example: An app that collects data from users might have the user fill out various forms. The content of each form can be specified by a data file, and the form built on the fly. Forms can be updated and new forms added just by downloading new data files.
When you do update your app, you can take some steps to at least not prolong the review process:
Make sure that you understand and follow the review guidelines. The guidelines are mostly common sense, and if your app clearly works within the guidelines and doesn't try to push the envelope it'll be that much easier for a reviewer to approve it.
Make sure that your app works. If your app is buggy, or if it looks buggy, it'll be harder to get it approved quickly.
Explain clearly what you've changed in the new version that you're submitting. You can write a note to the reviewer if there's anything special they need to know, and you should also make changes clear in the description that's displayed in the app store.
Anticipate busy times for the reviewers and either submit early or plan for a longer than usual review. For example, update your app early when a new iOS version is available so that you can build against the GM and submit as soon as Apple starts accepting updates for that version.