If the internal drive is getting close to being full, that can cause sluggishness. The OS and apps use the free space to write temporary files, such as the "swap files" used by virtual memory, "log files" used by the system, and "cache files" used by the web browser.
Once the free space becomes low, the remaining free space becomes more and more fragmented into tiny pieces, so even if you have a few GB's left, access to that space becomes more and more inefficient, which causes the performance loss.
You can check the current available free space by doing a Get Info (from a right-click) on "Macintosh HD" in Finder or by running Disk Utility. At least 10% of the total capacity should be free space (more would be better). A good place to look for files that you may no longer need (and can be trashed) is the Downloads folder. Be sure to Empty Trash to free up the space.
Speaking of Disk Utility, if lack of free space is not the problem, you should select "Macintosh HD" in the Disk Utility sidebar, and go to the First Aid tab. Use Verify Disk and Repair Disk Permissions. Don't be concerned if a lot of messages appear for Repair Disk Permissions, but if Verify Disk reports an error, you need to resolve that problem as soon as possible (and it may be causing the performance issue).
You can post back if that happens... or if your performance issues are otherwise not resolved.