How much should I spend when I send BitCoin to other people using electrum wallet?
Does this make sense to you?

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low/slow........high/fast   

The further left you move the slider the lower the fee you pay and the longer it'll take to confirm. The further to the right you move the slider the higher the fee and the faster it'll confirm. If you still don't get it go to tools > preferences > fees tab and check edit fees manually. Now on the send tab you'll be able to see the whole calculation including the absolute amount of fees you will pay for a particular transaction.