Hm. Good question. I’m not sure what would be the best region for your app. It depends (as always), on what you’ll be doing.
If you’re uploading a lot of media, I’d pick a region closest to the uploader. So if it’s primarily your client who will be using the app, I’d probably default to ap-northeast-1 (Tokyo). If you’re uploading from multiple regions, the simple solution is just pick a region and take a performance hit on the other regions, but if you’re worried about performance, look into edge-optimized API endpoints (Choose an endpoint type to set up for an API Gateway API - Amazon API Gateway). These are the default for REST APIs. They are probably the best solution if you’ll have “clients” (i.e., uploaders) in multiple regions.
Keep in mind, though, that if the client is “consuming” the products of the Transcribe API, i.e., the transcripts, it mostly matters where you store the transcripts. So if you want fast load time of the transcript data, use a bucket in the region of the client. In other words, starting the jobs may not be the bottleneck in performance; if the client is consuming the output, that may be what matters most.
Hope this helps!