If the only ones running at full speed are the closest to your lava tank, Id guess you need more connections to the lava tank. Liquiducts can carry a ton of liquid, but are limited where they connect to the tank. I'm running 24 magmatic engines and they required 2 connections to the tank even though one central line feeds them all. Both connections feed a single pipe and this allowed mine to run at full power. Steam works the same way. You need 9 connections to the boiler to get max power to 18 steam engines even though a single liquiduct can carry it all to them.