When I play multiplayer on minecraft, oftentimes during those hated lag spikes, you here people saying, "LAAAG" or things like that. Whenever I hear that, I can't help but ask sometimes, "Do you have optifine?" Sometimes I hear a yes, some say no, and some people just don't know what optifine is, as famous as the mod is. Optifine is a mod that was made to give to reduce lag, but it also gives you more options, hence, the name optifine comes from, or so I think (Opti = options, makes your options fine).
The first time I used this mod, I noticed a large improvement, and I still do. Optifine's options are very useful, and have some things mojang should add, like being able to turn animations off. Whenever a new version comes out, it gives you a nice little reminder every time you join a game in chat. It's only visible to you of course though, so you don't have to worry about servers getting meaningless messages.
Optifine is a great mod, without any downsides to it, it doesn't increase your lag (and it shouldn't, or else there would be no use of it) and gives you more options to the game. The post is somewhere in the forum, but I'm too lazy to find it :P but in the forum it has a vote for the improvement you faced. More than 70% noticed improvement.