Function Call cheaper on second call

Question

I have a couple of different smart contracts (Beacon Proxy etc) that can receive ERC20 tokens and performs various calls (for implementation and admin) and then moves all the ERC20 balance to a provided destination.

I've been trying to find a way to save on the gas required to perform the functions (and struggling) but one thing I've noticed is that the same function call is cheaper after it's been called.

I've been trying to investigate (and google search) for answers but I'm unable to piece it together.

Just wondering if someone would be able provide some reasons as to why the gas cost is reduced?

Is it to do with sload?

Example of the gas cost on a Local Hardhat forked from Mainnet using USDC.

First call
┌──────────┬─────────┐
│ (index)  │ Values  │
├──────────┼─────────┤
│ moveGas1 │ '83979' │
└──────────┴─────────┘
Second call
┌──────────┬─────────┐
│ (index)  │ Values  │
├──────────┼─────────┤
│ moveGas2 │ '66879' │
└──────────┴─────────┘

Answer 1

Welcome to ES! :) the SSTORE (write data to storage) opcode costs 20000 gas when assigning a non-zero value to a previously zeroed out slot, and 5000 in every other cases. I can't say for sure without looking at your code, but that's very probably why your second call is less expensive :)

Answer 2

Reusing previously used storage slots can indeed save gas.

For example, reading a previously read value from storage costs 100 gas instead of 2100. Also, a previously touched storage location uses less gas when written to. There are additional gas refunds when a previously filled storage location is emptied.

You should be able to use the evm opcodes calculator to find out how you ended up with the gas costs in your particular example.