When does Numbers sync spreadsheets?

Question

In Numbers, connected to wifi (good bandwidth), after modifying a spreadsheet I go back to "Spreadsheets".

Then I wait for the Arrow ￪ to appear, as it indicates that Numbers is syncing with iCloud.

However, sometimes

• it is instantaneous (does the sync immediately)
• it takes a couple of seconds
• it takes 10~30 seconds, or (rarely) more (stuck?)

So, what does the time it takes to sync depends on? Is there a way to configure that?