have you tried to adjust the FPS options to "Auto Adjust"?, if your computer is indeed old than try to have a look at the fps options and also try disabling the sound.
Check your process with "End Task" manager and depending on your operating system, Win XP should be running at most 32 process and at least 28.
Plus could you give me the specs of your computer by clicking on "Start" and than right click on "My computer" or "Computer" and click on properties in the drop menu that appears, and what operating system are you running?

I am getting a similar problem which I would describe in much the same way, my character and others (minions too) will move slowly and then "slide" and catch up to where they should be. Furthermore my cooldowns will finish but still not be available (cooldown timer clears ability still cannot be used) for a longer time afterwards relative to the length of the cooldown (i.e. a 10 second cooldown turns into a 12-13 sec cooldown and a summoner spell goes from 120 sec or whatever to more like 130 or 140 sec cooldown).

I have tried messing with the FPS options and disabling wait for V-sync. Anyone else have thoughts on this?

My computer is not old.

I am running a GTX 470 a new I5 processor and 4 gigs of ram.

I messed with more FPS settings and now the problem is maybe a little better but the ranged minion magic balls now streak across their path they look more like constant laser attacks than individual balls now.

Just played my first game today. Did a normal 5 queue and me along with 2 of the others on my team were all experiencing this.

My ping on vent never went above 50 and my ping in game claimed to be between 30 and 32 the whole game.

I am also getting this lag. My client ignores framerate cap. and goes far beyond its normal given. My latency runs at 90fps. Everything moves slow motion. Abilities don't cast when off cool down. My character will jump" from one part of the map to another. My computer is an Asus G73JH. Windows 7 64 bit. I have an ATI Radeon 5870 HD graphics card, an i7 processor, 8gb of ram, 3gb ddr3, and much more. Any help?

Also, the lag seems to be applying to the game clock. I saw it jump dramatically. Oddly, the chat seems to work just fine and always goes through immediately.

+1 same thing for me