I am having a similar struggle with mine. I'll echo what other's have said: make your spawnpoint really engaging and informational.

There are so many servers to chose from, sticking out can be tough but is necessary!

Good luck :-)

Find a unique selling point and make this the main feature of the players initial interaction with your server, if you cannot gauge there attention in the first 60 seconds, then they will leave.

If you need any help or support, PM.

Best of luck, and dont give up!

Quick joins and departures might indicate a lack of initial engagement. Consider a welcoming spawn area with info, clear rules, and maybe a starter guide. Encourage community interaction and events. Ensure server stability and performance. Test the player experience and gather feedback. Best of luck with your SMP server!

Speedrunners are so fast that they speedrun your server in 10 seconds

Based on the server ad you put up ( www.planetminecraft.com/forums/minecraft/servers/come-join-feathersmp-a-brand-new-semi-vanilla-public-smp-server-smp-678292/ )
The "Server shop & Auction House" would be expected to be at spawn & clearly identifiable
The "…friendly and welcoming community of players…." might be leading to expectations of a populated server.

Also the twist mentioned in " …vanilla Minecraft multiplayer with a twist" would lead me to expect something (the classic Wall_o_Signs at least) explaining the twist as well as other bits.

Even so, if 10 sec is accurate, it would seem players are leaving more or less based on first sight which does seem odd.
Have you been able to talk to any of those players about why they left so quickly? (I'm wondering about something like a very slow ping.)

I joined it, it just looks like every other survival server out there. A lobby with NPC's and crates, nothing new or unique to make people play your server over the thousands of other exactly the same servers out there.