As nice as it is to visually select your favorite sites, some of you have probably noticed the increased process and network activity that occurs when Safari is first launched and Top Sites appears. Symptomatically, Safari will be initially be slow to respond and load sites. In other words, our speedy Safari 4 takes a bit to warm up. This can be frustrating for a browser that promises top performance.
To facilitate Top Sites and ensure multi-tasking, Apple has created a completely separate process known as "Safari Webpage Preview Fetcher" to actually pull down the previews for your Top Sites. This is viewable in Activity Monitor.
The more Top Sites you have (using the "small" view for example), the longer it will take for this process to pull down the information. This process will disappear once it has finished its task. However, while it is running it will take about 25% of the CPU resource (Mac Pro 2.66Ghz Woodcrest) and about 300k in download bandwidth (DSL 3.0Mbits). Closing the Top Sites page in Safari will not close the "Safari Webpage Preview Fetcher" process as it will run in the background until it has completed its initial tasks.
Recommendation: For those looking for the absolute fastest Safari 4 experience, don't use Top Sites and definitely don't have it set as your default New Window page.