Stupid newbie question

Not a stupid question! We all have things to learn!

Dorcas is indeed extremely slow. I have -spd Dorcas and he might be literally the slowest lv40 in my whole barracks of 440-ish members. He's not much good on player phase unless I charged his ignis during the previous enemy phase or there's some squishy blue mage hanging out where she shouldn't be or some damaged opponent.

Try using more enemy phase play in the PvE maps! If you have access to the sacred seal forge, make yourself QR1 immediately and give it to Fjorm. Another option if you can get a 4* Subaki is to give QR2 to L.Ike. A team of Fjorm, L.Ike, Dorcas, and B.Lyn will clear a lot of the earlier content. (If it's not working, bring a healer.) B.Celica is also good if you use Xander to dance her, but then I would also bring B.Lyn so you have more options.

There are a few ways to combat against one of your units being slow, even less ways if they are an Infantry unit such as Dorcas.

Quick Riposte is the go-to skill for units like him. The chances of him having enough speed to double an enemy is fairly low, so Quick Riposte is a way to remedy that. Luckily he already comes with it, so you don't have to do anything as far as Skill Inheritance goes. He's a tank, so, most of the time, he'll want to bait the enemy to attack, take little damage, and then retaliate while attacking twice thanks to Quick Riposte.

As far as team building goes, there's nothing really wrong with the team that you have, although I do recommend getting a mage. Lilina can deal some serious damage, but she can be a bit slow, so she, most of the time, relies on OHKO'ing units.

You have Maribelle who comes with one of the most powerful Staff skills in the game, which is Dazzling Staff. If you have enough coins, you can give her the Wrathful Staff refinement to give her the ability to attack units at full strength (Staff units deal reduced strength without Wrathful Staff) without the fear of being counterattacked by mages, dragons, or units with Distant Counter.

I definetly recommend taking the advice from the answers above, but in the future I also recommend looking up videos for some tips as well. But I can show you how I've built my own Dorcas (don't mind the ss support). Vantage and QR work well together on a DC unit, so that's kinda what the build is based on. He can take care of greens and blues with no problems, but he struggles a bit against reds, but when he has vantage he can get rid of red mages with lowish defense easily as well. I hope you'll figure out what to do with your team ^^