At one of my previous jobs I had a desk at a major homebuilder's design center. After someone bought a home they would go through and pick out their appliances, carpet, tile, etc., and end with me. I had 50 minutes to design their builder-provided front yard and try to design and upsell the backyard. You either got good at being fast or you ate a lot of ramen.
As far as being accurate, ask the homeowner for a copy of their survey plat. Take it to Staples and enlarge it on the photocopier till it's a useful scale like 1/8"=1'-0". Trace over that on a clean sheet and you have the house, driveway, front walk, fence, and property lines. All you then need to locate and add to the drawing are any relevant trees or hardscapes, doors, and windows.
If you're charging for your designs then you probably want to make them a little more impressive. Otherwise, all you need to do is show what you're proposing in a clean, understandable way that'll sell the job. The black and white sketch was for one of several "pocket" perennial gardens the client had around the house. I knew I'd be making changes in the field anyhow, so why bother putting a lot of time into the drawing? As for the color drawing, it was for a winery client I was trying to get more work from, so I made it a little fancier - but simple circles with a circle template and no color would've gotten the job done.
Don't get me wrong, the software is great and the last time my computer was at the shop for a week it was killing me. But take a look at your sales process and make sure you're doing what will make you the most money in the least amount of time.