How much RAM do you have?
How much free space on your hard drive?
If Safari is getting very slow:
(Presumably you regularly empty your Safari cache by deleting the following file:
and clear your History)
Adding Open DNS codes to your Network Preferences, should give good results in terms of speed-up* as well as added security, (including anti-phishing and redirects) (Full information about Open DNS is here: http://www.opendns.com/home-solutions and here: http://www.opendns.com/technology/opendns-vs-google-public-dns/ )
and further independent information can be read here:
but results may vary depending on where you are located.
If you are using a single computer: Open System Preferences/Network. Double click on your connection type, or select it in the drop-down menu, and in the box marked 'DNS Servers' add the following two numbers:
(You can also enter them if you click on Advanced and then DNS)
Sometimes reversing the order of the DNS numbers can be beneficial in cases where there is a long delay before web pages start to load, and then suddenly load at normal speed: