My dual flush toilet fills up slowly how do i fix

A dual flush toilet is a fantastic option to save water and make your home more environmentally friendly. Giving you the option for a half flush or full flush at your discretion, you can lower your water consumption while still getting the job done. However, like all toilets, they can present problems that mean the toilet doesn't function as it should. One common problem in dual flush toilets is constantly running. You hear the tank constantly being filled, even when it should be finished and completely full. If your dual flush toilet is constantly running, first check for the top three problems that cause this trouble:
Flapper or Seal Trouble
The flapper or seal is often the culprit in constant running water in both normal and dual flush toilets. Over time, this rubber piece may become brittle or simply misaligned. To check it, turn off the water supply and flush the toilet to completely empty it of water. Remove any remaining water with a sponge or towel. Then, you'll need to remove the flush valve canister in order to access the seal. Once you've found the seal, check to see if there are any cracks in the seal. If there are, you'll need to replace it. Otherwise, check to see if it can be re-aligned to sit nicely in its place. Some homeowners find that applying a bit of Vaseline to the seal can renew it, making it supple and functional again for a while at least.
Debris
Sometimes the water supply isn't as clean as we think. Or, dust and dirt may be able to enter the toilet, causing the fill valve to malfunction. To check it, turn off the water supply and remove the lid from the fill valve. Then, look for any debris and remove it. You may also consider brushing out the valve with a small brush. Once you're certain it is clean, recap the valve and check if the problem has been solved.
Broken Zip Tie
The fill valve in most dual flush toilets has a zip tie to keep the base of the fill valve in place. If the fill valve isn't strongly secured, it can cause the toilet to run. Sometimes these ties wear out or break before the rest of the device does. You'll need to replace a broken zip tie with a new one if you find that it's broken. If possible, you can place two zip ties on the fill valve in order to assure that the base doesn't wobble, causing the leak. Many times these problems are easy for the average homeowner to check for and repair. In the case that you are not able to find a solution to your dual flush toilet's constant running, then you'll need to call in the help of a professional plumber. A high quality plumbing company will be able to help you troubleshoot your dual flush toilet troubles and make sure it's functioning properly again.

check /replace the small rubber washer at the end of the fill tube as they tend to grow with age and reduce the flow of water past the inlet

allow tank to fill a little more by adjusting fill valve. When there's not enough water flowing from tank the stopper can over-power the water flow and cause the flush to stop by re-sealing.

I would ckeck to see where water fill line is inside toilet tank and then adjust flush valve to try to get it to shut off at this point. Not knowing whether your toilet still has original flush valve it is hard to tell you how to fix it. If it has float mounted on rod to flush valve there is usually a set screw on the top of the flush valve in the center. Turn this screw clockwise to make valve shut off sooner. Try to set it so that water shuts off when it is at fill mark. If float is mounted on brass float rod you can support rod closest to flush valve with left hand and carefully bend rod down a little with your right. This will cause flush valve to shut off sooner. If the seat is worn to badly in flush valve you will need to replace it.

The ball float follows the tank water level, and repeated cycles of the float dropping and opening the fill valve would likely reflect a leak in the flush valve. You could try adjusting the float arm downward so that it will respond to a lower water level. Try turning off the supply valve, flushing it and listen for water leaking past the flush valve after it closes. If it does, replace the flapper or adjust the chain.

No there isn't. It is set by the design of the bowl and the depth of the p-trap inside the toilet

Check your angle stop ==valve underneath close to the wall see if it's open all the way if it is and you still have the same problem then go ahead and shut it off, disconnect it from the toilet and the valve and see if there is anything stuck in the hose (a gasket maybe) if nothing blocks the hose then. Reconnect the hose starting from the valve put a bucket on the other side to catch the water and open the valve as if to clean the hose. After you do that reconnect the hose and try again if still does not work properly then you have to replace the fill valve (the tower looking stuff inside the tank)

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You need to replace the tank flapper valve gasket.