How much does it slow it down? It seems much easier for me to manage segmentation via the API using custom fields rather than moving people between lists, as all I have to do is update a single field to move subscribers between campaigns. Plus there is the issue of sending campaigns to multiple subgroups of subscribers, which is easy when using segments to separate subscribers, but when using lists to separate subscribers, it results in merged lists (which sounds problematic).
If there is a better way to handle segmentation, I'm open to hear about it, but it's worth a little performance hit otherwise. I'm using an external SMTP service to reduce server load, but if the use of segments is a potential future problem then I'd like to know. Handling segmentation is an important part of how I'm using mailwizz, so I'd love to hear any better ideas.