You're talking about "paging", a common technique that Windows or OS X use to allow you to run big programs or work on large files (e.g. video editing) that takes more storage memory space than you have installed in RAM. Installing more memory will decrease the amount of paging, so decreasing the amount of HDD activity, and draining much less of your battery power. Reading and writing to HDD involved mechanical spinning a stack of multiple ceramic/metallic disks and moving the read/write heads back-and-forth. Easy to appreciate this takes much more power than reading and writing to memory (more like reading/writing to a USB key). Here are the things to try, as needed, in the order listed to improve your MBP's speed when browsing:
1. In Safari Preferences/General, reduce the History item retention to only as long as you need to. If you use Add Bookmark and Add to Reading List for stuff you want to revisit later, you don't need to keep your browsing history for long. Set "Remove download list item" to "Upon successful download" or any other seeing but never Manual to remove junk within a day. Along this line, I run the free version of CCleaner every time I'm done with browsing for 2+ hours on my old MBP.
2. If you're using OS X Yosemite or earlier, boot to Recovery Partition (press and hold down Command and R) after hearing the beep on a Restart) and use Disk Utility to Repair Permission. I find benefits in doing this monthly on my 2008 MBP running Yosemite. I also run off ethernet from my router rather than Wifi when browsing heavily.
3. Maximize the amount of memory on your MBP. If you have a Late 2007 MBP, you can upgrade it to 6GB with an upgrade kit from OtherWorld Computing
4. You'll see an immediate speed benefit on system boot after upgrade to SSD drive. After that, human think-time and network bandwidth is the overwhelming factor when browsing.
I successfully upgraded my Early 2008 MBP to 6GB ram with this kit before I install OS X Mavericks. I currently have OS X Yosemite installed on this Mac and is still happy with it. My HDD is only a 5400rpm drive. I found it overheated with a 7200 rpm HDD and ruined a new battery within 4-6 months.