The fee slider in static on Electrum have these fee rates: 1, 2, 5, 10, 20, 30, 50, 70, 100, 150, 200, 300 sat/vbyte.
If you compare it with another wallet that can make you to use the fee slider to customize the fee, it will look like this: 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 and so on. Few wallets even give you decimal like 1, 1.5...and so on while using the fee slider to set the fee rate. The two wallets I saw fee slider used for accurate fee customization are Sparrow and the no-more-existing Samourai.