I just banged up my bike, what damage should I look for?

Start with a basic M check. Look for both damage and functionality.

The original M check is https://www.sustrans.org.uk/our-blog/get-active/2019/everyday-walking-and-cycling/the-m-check-for-your-bike-in-11-steps

Essentially you methodically examine and test every part of your bike, starting at rear wheel, progress up to saddle, down to BB, up to headset, and down to front wheel.

You methodically test that every part works as it should, makes no bad noises, and is not malfunctioning. You also check for loose items that should be tight, and tight items that should be loose.

If you have access to a workstand, or some other way to hold the bike, then its a bit easier to work through all the gears, but this is not required. You can either hold the rear of the bike off the ground, or hang it by the saddle from something convenient like a tree, fence, cord, or similar.

Another viewpoint on the M check is at https://bikethere.org.nz/im-riding-in-traffic-free-places/m-checks-and-handy-tools/

Specific to your description, you "slammed your bike" and then dragged it through undergrowth. So something happened to make you fall off, and then you wheeled the bike through some vegetation.

Part 1 - falling off. Figure out why you fell off. If you hit a rock or pothole with a wheel that stopped your forward motion, then check all around that wheel/rim for damage. If you overcompressed your suspension and hit the ends, then check for air loss (if air based) or that the resistance hasn't significantly changed. If some part of your bike was the cause of the fall, then check that part.

Part 2 - undergrowth. Not sure if you rolled your disabled bike on its wheels, or if you literally dragged it sideways while the bike was lying down. Either way, shouldering the bike would have been a better way to move it.
Check the whole frame and transmission for bits of grass/twigs/stones and removed them. Give the bike a good wash to remove grit and dirt that will increase wear.

Part 3 - it runs slower. That means something's rubbing. The prime candidates are

• tyres rubbing on frame/mudguards
• brake pads not quite opening enough
• transmission not quite in line (ie something's out of position)

The cause should be pretty obvious once you find it.

It often helps to clean the bike before or while checking, to help remove dirt that could obscure damage.

If you have any areas that seem suspect, do take some clear photos and edit them into your question.

Shouldering a bike to carry it:

Essentially putting your right arm through the frame. This keeps the oily transmission away from your body. Some frames may not have room through the frame, so you can hook the nose of your saddle over your shoulder instead. A long walk may require some padding between your shoulder and the frame.

^{By Original uploader was Julius.kusuma at en.wikipedia - Transferred from en.wikipedia to Commons., CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=3778736}