Almost every one-click-hoster provides a Free-download mode and a premium-downloadmode. For many hosters, JD supports both modes.

This is the default mode. The free generaly comes with various limitations.

The user can only download a certain amount of data e.g. 100mb every 10 minutes. Afterwards his IP gets blocked for a certain time. He has to wait.
JD can perform a reconnect to get a new “unblocked” ip. After the reconnect you can continue loading. See Reconnect

Some hosters want users to wait before the download starts. It depends on how the hoster implemented this wait time. Sometimes JD can bypass it, sometimes not.

Almost all hosters limit the download speed in free-mode. Depending on how busy the hoster is, the download speed may be very slow sometimes.
The maximum download speed is always limited by the hoster, or your internet bandwidth. It's extremly unusual, that external software, like firewalls, or security software can cause download speed limitations.

In freemode, the hosters often show ads which can be bypassed, either by using a browser with and adblocker addon or JDownloader.

All those restrictions are usualy not present in premium mode. Most hosters sell so called “Premium Accounts”, which cost between 5 and 10 € /Month. There are many advantages in premium mode.

Usualy you have a better download speed in premium mode.

You can download multiple files at once.

Chunkload: More than one connection/file boost your downloadspeed

Most Hosters support resuming interrupted downloads in premiummode.