Slow upload speed (but only to Dropbox). Dropbox is now unusable

[Verenhaul]

I've begun to experience this recently:

1: Uploads to dropbox are (as of about one week ago) consistently around 0.14 Mbps. This happens through desktop or web interface.

For perspective, that's a 300 baud modem from 1984 kind of speed.  It renders me unable to work using dropbox.

2: Speed tests of the line show consistent ~75 Mbps downstream rate / ~20 Mbps upstream rate.

3: If I try uploading the same file(s) to google Drive (or anywhere but dropbox), speeds are as-expected.

4: I have re-installed the dropbox app.  Prefs for everything, including Bandwidth, are at default, so please don't suggest it as a solution. 
 

Dropbox is the only destination exhibiting the problem.  Any help appreciated!
What kind of tests can I do to narrow things down?  Thanks in advance.

(UPDATE EDIT: 9 Nov)  It's not a fix, but a good clue: I have seen immediate return to expected upload speeds after connecting through a VPN. 
Another user below has also found respite by altering the MTU size in their router configuration.  Will update with new findings.

POSSIBLY suspecting a bad hop somewhere along the route between me and dropbox.  If it is, then How to remedy?

[Megan]

That's what we like to hear -or read in your case- @OrsonKane!

Glad to know that everything is back on track. If you need anything else, you know where to find us!