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Experiencing some sort of symmetric bandwidth throttling. Is there a max bidirectional and simultaneous data transfer cap?

E.g.
Downloading file from Dropbox, where they cap downloads per connection at 10mb/s. Was getting this full 10MB/s, though once I started uploading a file to Google Drive the download speed for Dropbox dropped to 500KB/s.

Any clarifications or insight on simultaneous data transfers?

Saturating your upload will have an adverse impact on your download. While we usually visualize download and upload as two independent actions, downloading requires a lot of packets sent out of your network to keep things going, which get bogged down when your outbound queue gets backed up by uploading large files. If you limit your Google Drive upload to about 80% of your upload speed connection, you shouldn't see problems.

What if google drive (or similar service) doesn't expose a bandwidth limit setting?

In Google Drive's "Backup and Sync" utility:

1. On your computer, click Backup and Sync
2. Click More More and then Preferences.
3. Click Settings and then Network settings.
4. Next to "Download Rate" or "Upload Rate," click the button option you want:
   • To use a slower rate, choose Limit to and use the arrows to change the rate.
     To use a faster rate, choose Don't limit.
5. Click OK.

The root cause of this is something known as "bufferbloat". If you are technologically adventurous, you can more or less solve it completely by using a special queuing algorithm on the router right before the bandwidth bottleneck, i.e. your Internet connection.

Routers from Ubiquiti (EdgeRouter, Unifi Security Gateway) have this feature built in. Ubiquiti calls the feature "Smart Queues". Other routers brands might as well, or it might be available with third-party firmware. The latest and greatest algorithms are known as "cake" and "fq_codel".

With this algorithm enabled, a single large upload will no longer completely monopolize your upstream connection. You'll still be able to download, browse, and use latency-sensitive applications (such as voice/video calling and online games) without noticeable degradation.

It would be good for this functionality to eventually be provided by the ISP's customer premise equipment, but for now, it's something we as consumers need to handle on our own.

I have this functionality enabled on all my small business and residential clients across multiple ISPs, and I can confirm that it is tremendously beneficial for maintaining the usability of an Internet connection under heavy load.