The quick solution is to use the Fast Draw theme to have Voyager draw the Charts in a lower-resolution mode. It's about 2 or 3x faster than the Sectional theme. When you're almost done planning, you can click Sectional or IFR Enroute to switch to a more detailed view. You can also have Voyager default to Fast Draw mode by selecting Options from the Tools menu and, in the Default VFR/IFR Theme drop-down lists, select Fast Draw.
But adding more RAM is the best, and still inexpensive, way. Although Voyager runs acceptably on most 512 MB systems, it runs considerably faster when it runs on a machine with 1 GB or more of RAM. The difference is more dramatic than you might expect and it helps not just Voyager but every other program on the computer. And, as of this writing, 512 MB of additional RAM is only about $50. The CPU speed is also important, obviously. Check System Requirements for more details (in a nutshell, we suggest a 2 GHz Pentium IV for desktop machines and a 1.2 Ghz Core Duo or Centrino for tablets and laptops). For laptops and tablets, the hard drive speed can also be a factor (esp. if RAM is low). Many laptops and tablets come with relatively slow 4600 RPM hard drives to save power or cost. Desktop machines typically have 7200 RPM drives (50% faster) although few laptops have this even as an option. For laptops/tablets, therefore, try to get one with a 5200 RPM drive (IBM/Lenovo ThinkPads usually offer this an option).