SMOOTH SENDY CAMPAIGNS
At the beginning of the send cycle, the load is extremely high (99%-100%) because it needs to process all the emails. Each email has links to process and various things that Sendy needs to do.
During these few minutes, if any clicks come through, there's a chance that Sendy will drop / not record those clicks. We have had some campaigns without issues, but roughly 15-20% of the time we see this sort of "stutter". We were able to eliminate the stutter by setting the send-rate lower - to around 1 or 2 per second should be a rather reasonable send rate.
Some of this is indeed trial and error, and servers & email are far from a perfect science even today. When you have a big campaign to send, set your send-rate lower and keep an eye out for "stutter" so your customers don't experience any negative impact.