Ethereum (ETH) transaction fees increase when the network is busier. This is caused by more people making transactions like sending tokens, trading on DEXes or depositing their assets to lending platforms.
A recent tweet by WeissCrypto states that:
According to this post, “The average fee per transaction on Ethereum reached $6.04 Wednesday night (August 12), according to Blockchair, the highest fees since 2015”.
In order to compensate for this, Trust Wallet will always use the recommend gas price to ensure transactions get processed as soon as possible. Lowering this setting may cause further delays or even cause your transactions to fail.
Even when simply sending ETH to another wallet, the app will follow the recommended gas prices. As shown on the following screenshots, sending $8 worth of ETH would cost around $1.50. The app will always use the highest gas price so that your transaction is prioritized.
Ethereum uses the “first price auction” system to price gas. The system works by having every sender submit a bid, known as a gas limit, for how much they’re willing to pay. Miners then pick up desirable transactions and include them in the next block. In theory, this allows senders to prioritize their transactions by paying a higher fee while saving ETH through refunds.
Source: Understanding Ethereum's Gas and Transaction Fees - Crypto Briefing
Making a more complex transaction like a token swap on a DEX or submitting a transaction to a smart contract also warrants a higher fee. The Network Fee that you will see on the app is just an estimate. This is the maximum fee that will be consumed for the transaction given the current gas prices. Not all of it gets used up and some of it gets refunded.
Here is a sample of token swap from one ERC20 token to another ERC20 token. The estimated fee was around $10 at the current Gas price of 55 Gwei. But eventually, the fee used was only 43.22% which is just under $5.
Note: If your transaction gets stuck due to a low fee setting, you can follow this guide to try and cancel it or increase the fees, Pending "Stuck" Transactions.