There are actually options you can add for exim configuration in the WHM > Exim Configuration > Advanced Editor area in the topmost box that would either reduce, suspend or control connections when the machine has a high load. I'm going to mention them for informational purposes in case you want to consider using the settings. The issue I see with a cron job to renice the processes is that they are short-term processes and the cron has it's own activity to renice, so you are actually trading exim process activity for a cron that might well cause load itself.
queue_only_load = #
This value will only queue mails when the load is at or below the value placed. Let's say you put 10 into the queue_only_load option, then when load is over 10, messages in the queue will wait for delivery until the load is at or below 10.
Another delivery load option is the following:
deliver_queue_load_max = #
Here, no deliveries are performed for queue runs when the load is above the number specified. The queue runner will check the load periodically and abandon delivery if the load is above that number.
Another configuration option you might consider is this one:
smtp_accept_queue = #
This is the number of simultaneous incoming SMTP connections that are accepted for the queue run delivery. The default is unlimited to allow immediate delivery for all messages, so you could limit the number allowed, then those connections exceeding the limit will be placed in the queue and left for the next queue runner.