The overhead is negligible with the prefetching browsers do, and even that is closing fast with things like TCP fast-open.

Plus TLS false start and TLS resumption are already here and work great for repeat visits to remove that extra overhead.

Performance isn't an excuse for not using TLS anymore.

Oh I agree it's no excuse, OP just asked what the tradeoff/cost was. There is still some but as you say it's negligible and especially so for a very small site like that.