Currently, for some unknown reason (what was I thinking?!) the
log_conversion.referer_name columns are limited to 70 characters. This can be very limiting because it is common for campaign names (for example) to exceed 70 characters.
I've just tested and it is not too slow, but it's definitely not fast to upgrade a column from VARCHAR 70 to VARCHAR 255. So we can't do it before Matomo 4.
Related bug we recently discovered: https://github.com/matomo-org/matomo/pull/13573