You say you do a mash of 3.5 gallons then 3 gallons of sparge to make up the batch volume you need pre-boil
If you are doing a mash consisting of 10#'s of grain and you put 3 gallons in the mashtun, your yield after sacch. rest when drained would be around 2 gallons of runnings.
This means you would need 4.5 gallons of sparge water.
What I would do, is tighten up your mash ratio like mentioned above then, take your full amount needed of sparge water and heat it up to around 185-190*F when you are getting toward to the end of you sacch rest. (time depends how fast you can heat the 4.5 gal. volume on your setup).
At the end of your rest..Now take a 1/2 gal. pitcher and remove 1.25 gals. of the heated sparge water, add that to the mashtun, and stir it in well. Close the lid and let it settle for a couple minutes. Now open the lid and start your vorlauf, starting with a very slow runoff when you start your vorlauf, then you can increase the flow rate after collecting about a quart. Vorlauf until the wort runs clear then drain. You should have 3.25 gals of first runnings now.
Check the temperature of the left over sparge water, making sure you are still up around 185-190*F.
Now, making sure your drain is closed, add the rest of the sparge water to the mashtun. Stir really well, and shut the lid for another 2 or 3 minutes, Then open the lid, and repeat the vorlauf like in the first step, Then drain those runnings.
Speed doesn't really matter after your bed set and the vorlauf has the liquor running clear. The higher sparge water temperature is more important, because it thins the viscosity of the liquor, allowing the sugars to wash more cleanly from the grains.(think on the lines of motor oil in the winter time being like a real thick syrup until the heat of the engine warms it up, then thins it out!)
Stepping Back Some.. Once you collect those first runnings..you can put those into the boiler as soon as it is freed up, and start towards the boil while you are still working on collecting the second runnings.(5 gal. hdpe bucket)
Once you have drained the mashtun for the second time, put a small bucket or container to catch any runoff left in the mashtun, Then just add 3.25 gals. of second runnings to the boiler. By doing it this way, it will speed up the time it takes to get you to the boiling point.
3.25+ 3.25= 6.5 gals pre boil should give you a pretty good extraction