A build cannot fight above its body’s weight limit. Even below that limit, attachment weight reduces movement: the base penalty is the total attachment weight divided by 40, rounded down. Body and support effects then modify speed.
Separate legality from performance
Carrying capacity is a hard limit set by the body. The machine also requires owned compatible attachments, mobility and at least one weapon. An incomplete or overweight draft stays editable in the Workshop but cannot enter battle.
Being legal does not mean being fast. The current movement rule subtracts floor(total attachment weight / 40) from the living mobility module’s base movement, bounded to at least one before other modifiers. Crossing a weight threshold can therefore slow a build even though it still fits.
Look beyond the mobility icon
Chassis bonuses, Thruster, Afterburner, pursuit behavior and phase effects alter effective movement. Enemy pulls, drag and knockback can make a healthy machine appear slow. If the mobility attachment is destroyed, movement starts from a much weaker baseline.
Turning is another source of lost attack time. A machine may reach the opponent yet need time to face it before firing. A body swap can change that delay even when the mobility part remains the same.
Reduce weight with a purpose
Remove or replace the least useful attachment for the matchup. If a heavy defense never blocks the damage that defeats you, a lighter option may let your weapons attack sooner. If you already maintain range, sacrificing durability for speed may give little benefit.
Test the result in battle and compare contact time, firing time and survival. Engineering cost is not the same as weight, and the current build validator does not enforce an engineering-budget ceiling. Use the actual Workshop warnings and carrying total.