There is no such thing as an all round speed unless you take the lowest common denominator being the biggest diameter piece of wood you will ever put on the lathe and set the speed for that. For example, if your biggest piece is 12" you could set the speed to 500rpm and leave it there (assuming everything you ever mount on the lathe is properly balanced). Then if you mount a small spindle of 2 or 3 inches, you will find the lathe is running far too slow to cut at a reasonable rate.
If the variable pulleys are causing problems, try either a strip down/reassembly with greasing all the moving bits or if it looks reasonable, a quick clean, regrease and exercise the pulleys to see if they move as desired. Remember this type of pulley should only be adjusted with the lathe running so it's important to set the speed to minimum before mounting a new piece of wood.