Storage space vs. number of files
The performance of the Dropbox application starts to decline after reaching about 300,000 files. This is a soft limit and depends highly on the hardware specifications of your computer. Some customers are able to have more files without issue. As a hard rule, we recommend never having more than 1 million files on any single computer. This limit is related to the number of files, not to their size.
If this appears to be causing syncing issues, you can use selective sync to select only certain files to sync to your computer, but still have them on dropbox.com and on other computers.
You can also adjust your bandwidth settings to help Dropbox sync faster.