Not working please help (currently user 2800)

Hi, 5 days ago I installed plugin when user reached 150 i send notification and it sent successfully. But, now my website user reached 2800 and sender notifications but, status shows running from more than 16 hours. So please help. Is there any setting i need to change ?

In one or two month my user will cross 1,00,000+ so also help me for how to send notifications more than 1,00,000 + users ?

(My current hosting plan : digital ocean 1gb ram, 1 vcpu, 25gb bandwidth)