Assuming you don't have an employee match, then:
If you put $6000 in an HSA via payroll deduction, you will actually drop your take-home pay by $4,500. You still pay social security and medicare tax on the HSA contribution, but not state or federal income tax. You put in $6000 but it only "Feels" like $4500 because it goes in pre-tax.
Instead, let's say you deposit a $6000 lump sum and immediately withdraw $6,000 to pay your bills. That costs you the whole $6,000 up front. BUT (and this is the important part) you take a $6,000 deduction on your form 1040. That saves you $1,500 in taxes. So you get a bigger tax refund in March than you would have without the deduction.
In other words, if you make payroll contributions today, you see the tax savings today. If you make a lump sum deposit, you see the tax savings later. But it is exactly the same dollar amount of tax savings in the end.
And more than that, there is a way to move the tax savings forward anyway. If you make a $6000 lump sum today, and withdraw it tomorrow to pay your bills, you can go to your employer and claim 2 extra allowances to your W-2. That will result in you getting the tax deduction in your take home pay instead of waiting until March.