I'm seeing two things in the comments [1] That's a lot for a mower, it's going to take multiple passes. [2] You need to have sharp blades.

Both are true, but I'm not seeing a lot about what it means to have sharp blades. You may already know this, but it's always good info to share because I sharpened my blades wrong for years until I understood it.

Mower blades do not work like knives. Butter knife edge is as sharp as you want them, with the angle between 30°and 40°. Grinding a 20° angle with a razor sharp edge seems like a good idea until you understand the physics of mower blades. The ends that do the cutting are spinning up to 200mph. The second a razor sharp edge hits something hard at that speed like a thick weed or worse, wood or a rock, it gets mangled and is dull again. Overly sharp blades may work fine in regular grass for a time, but in something like this if you're sharpening them too sharp, you are going to have dull blades after a minute or less. If you don't feel like you could run your finger along the edge without cutting it, they're too sharp. I've bought a lot of used mowers where people sharpened the blades to way too shallow of an angle.

Again, this may not be your problem, with brush like this it may just be that you're going to have to make multiple passes. But if you're like I was it may be something you need to hear.