One solution is to use Push Notification Fixer (PNF). The delay of push notifications (whatsapp included) is due to the connection being closed by the carrier after some time of inactivity. The phone, in order to keep it open, sends periodically an heartbeat (basically a small package of data), but the default time between two heartbeats is often too high because the carriers close the connection sooner. With PNF you are able to reduce the time between heartbeats so that the connection is kept open.
--- DegeNERDation in progress. ---