I saw a few thread saying speed is a useless stat (compared to health/strength/def at least). Wondering what is the optimal value for speed? Like at what point is enough and I should stop considering adding to it?

It comes down to personal preference but people tend to stop at around 6000-7000 speed since most of the strongest opponents have these speed stat and you can boost your speed stat in one turn using two different weapons at the same time. I like the idea of having 7001 speed so at least you can outspeed para saggitarius after the boost.:P

Thanks for the answer! so it's probably not something I should be worrying about at my stage xD I was partially wondering if I should just not input anything into speed rn and I'm only at tier 3

Speed generally becomes important when you are in higher tiers and when you are using weapons and/ scrolls that scales with speed. People tend to advice new player to have a even build with their first pet. If you have really high health and strength, having nearly no speed works too so it still comes to personal preference. However, you can always leech too using 1 medium token from the battle shop to adjust your stat in the future if you want.