When using a free gmail account to receive emails via Pop3, there is occasionally a lag.
With a Pop3 setup that uses gmail as the client, your email is received at your server, and gmail is connecting with the server and pulling the emails into your gmail account.
This lag is a result of gmail not syncing with the server often enough.
It is especially common with new accounts, and usually gets better once gmail figures out the optimal syncing schedule based on the frequency of new emails the account receives.
For a brand new email address that doesn’t receive tons of emails, the sync takes place less often in order to conserve resources.
The good news is that you can instantly sync your gmail and import all emails via the settings > accounts and import tab > check mail now option.