Hi everyone, this is Andy here:

In the past couple days, there were reports from members of the community about performance issues. After some investigation, we discovered that the issue was mostly impacting users who are located further away from Switzerland where our main datacenter is located.

That's an important hint, and we have seen this phenomena before. For Proton VPN, we actually built our unique VPN accelerator technology to overcome this: https://protonvpn.com/secure-vpn/vpn-accelerator

So last night, Proton VPN engineers looked at the problem and a solution was discovered. Proton Drive is different from Proton Mail because it is designed to handle large files (individual file sizes you can upload/download is actually unlimited), and as a result, uses a new technology which we posted about here: https://proton.me/blog/proton-drive-web-encryption-technique

A detail not mentioned in the blog post is that to keep memory usage low and performance high despite the usage of end-to-end encryption, Proton Drive uses a 4MB "chunk size", e.g. it splits large files into 4MB pieces. However, Proton Mail's infrastructure is configured to require clients to reconfirm with the server after 64Kb has been uploaded and this configuration was re-used for Proton Drive.

Each reconfirmation requires one connection round-trip, which means the further you are from Switzerland, the more it impacts you. This issue obviously doesn't impact other cloud storage providers who use cloud servers located around the world, which is better for latency, but worse for privacy.

Given the 4MB encrypted chunks we use, the 64Kb setting was far too low. So we can overcome the latency issue by increasing this and massively reducing the number of round-trip connections required.

The fix has now been made and the results are immediately apparent. By the way, this was also slowing down Proton Mail as well, but given that emails are usually much smaller in size, it wasn't really noticed. Below is the median and 95th percentile upload speeds in MB/s (in bytes not bits):

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Median and p95 upload speeds per chunk

You can clearly see the difference when we reconfigured the infrastructure. Btw, in the above plot, it's important to note that this chart is showing median speeds per chunk. The actual speed experienced by the user is many multiples faster because our clients are designed to upload multiple chunks at the same time for each file, and also multiple files at the same time, in order to maximize performance.

This is a good example of how operating a number of services is beneficial for the community. We used expertise from Proton VPN to resolve an issue on Proton Drive reported by the community, which was unknowingly also impacting Proton Mail.

Please keep your reports coming. We take them all seriously, are committed to communicating transparently, and are always working to meet your needs and expectations.