It's a run-on sentence, so it'll need significant restructuring. I think it's better to split this into two sentences. "It can take a while for an application to start, because we need to build all the dependencies and deploy it on multiple drones (when specified). Therefore, we start applications in the background, and notify you when the application is started in the Application page." I might also make the last part of the first sentence "and potentially deploy it on multiple drones, when specified."