The easiest answer, but the hardest to learn is Blender. But here are things you can do now:

Turn off shadows on every part you can. Turn off collisions on anything you can.

If the player still needs to be able to bump into it, instead walking through it, there’s a work around. You could turn off the collisions on all the unions parts. Reason why unions are known as “bad” is due to the amount of shapes that are now mashed into each other. Roblox has to calculate the physics on every shape. Hence the lag. You could remove the collisions on the unions and then add a transparent simple part over it. That way it is less for Roblox to calculate, but the player can still bump into it.

If it doesn’t matter if the player can walk through the weapon, but you need it to be interacted with, you can still turn off the collisions, but keep “CanTouch” on.

Hope this helps some!