It appears you're using our Remotely Hosted Downloads feature, so one thing to bear in mind with that is the first download of any given product's file will be streamed from your remote server, to our server in Tucson, Arizona, to the buyer's ISP and computer. During this initial download, we also retain a copy of the file in our cache and sync another copy of that file up to our Amazon S3 provisions for long-term storage and delivery.
Subsequent downloads of the same file would be delivered directly from our cached copy on Amazon S3, after we first check your remote file URL to confirm our cached copy matches the most current file at that URL, so these downloads should offer the best download performance available.
I presume you're located in the UK, since your pricing is set in GBP. If your remote file storage is on a UK server, and the first downloader of a product also happens to be in the UK, that initial download would be making two transatlantic hops -- first from your remote server to our Tucson datacenter, then again from there back to the buyer's ISP in the UK. This extended transmission route would likely affect performance adversely for that initial download.
Furthermore, early downloads after the initial one and before we get a copy of the file synced up to S3 (or in the unlikely event of an outage at Amazon S3) would be served from our Tucson datacenter, which may result in less than ideal download speeds, as we need to throttle the bandwidth allocated to such downloads in order to keep the rest of our service available to the Internet at large -- i.e., we can't allow these early downloads to saturate all our servers' available bandwidth, as that would make our site, Admin panels, and Cart service unresponsive.
This help page covers some other factors that may contribute to slow/stalled downloads in general: