I just wanted to know because my computer is very old. Please don't say things like "turn your render distance down" and things like that.
Is it general lag all the time or do you only notice at certain times? Are you giving Minecraft enough memory to work with? As suggested, optifine can do FPS enhancements by giving you the ability to fine-tune graphics options. Lag can be caused by too many mobs bumping into each other. You could try killing excess mobs in farms or giving them more space to move freely.