Adding some more details to Ciphertxt answer:
Throttle
You can check the default throttle setting by running the following command (assuming there is no changes to the throttle setting before):
stsadm -o getproperty -pn workflow-eventdelivery-throttle
You can change the throttle setting by running the following command, in which you can replace 25 with the new value:
stsadm -o setproperty -pn workflow-eventdelivery-throttle -pv "25"
Batch Size
You can check the default batch size setting by running the following command (assuming there is no changes to the throttle setting before):
stsadm -o getproperty -pn workitem-eventdelivery-batchsize
You can change the batch size setting by running the following command, in which you can replace 125 by the new value:
stsadm -o setproperty -pn workitem-eventdelivery-batchsize -pv "125"
You can do it at your SharePoint Server either by yourself or a SharePoint System Administrator.