The compression will definitely take some time, but that big of a difference is much larger than I would have expected. On my "GreatestShow" sequence, it goes from 1.8 seconds to 8.2 seconds, but the size drops from 1.1GB to 93MB.
That said, it's something we can easily change. The zstd compression library supports over 40 different compression levels. The version on the Pi/BBB supports levels from 0-25 and the version we use for xLights supports negative levels down to -20 or so. Currently, the default is level "10" as that seemed (at the time) to be a good balance between compression and speed, particularly on the Pi/BBB I was testing it on. However, it may make sense to drop it lower. For example, dropping to "1" drops the save time down to 2.1 seconds, but increases the file size to 104MB, so 10% larger. On the flip side, increasing the level to 15 makes it take 35 seconds to save, but only drops the file size to 89MB.