...when I load a replay and try to seek/skip to anywhere in the timeline it takes ages or not at all.

I can increment the slider gradually and that works but takes so long...

My PC ain't a monster but usually handles this stuff with moderate ease...anyone know whats up?

I hate it when browsing a Replay, that it reloads the whole thing.
This isn't that bad with me since I have a "powerful rig".

You might have installed something lately which takes a lot of CPU/RAM?
Look for processes running in the background.
Maybe a sector of your HDD is corrupt and the replay tried to write/read from there. (probably not)

SPR or MPR? For MPR it should be really quick, no more than 2 seconds even in long replays.

When you skip forward in a replay (using the timeline at the bottom of the screen) what happens is that all of the inputs saved in the replay are processed but the graphics are not updated. This is the reason why it can sometimes take a long time to skip through a replay. For a long replay with a high number of cars (e.g. a 1 hour race with 25 cars) it could take a significant amount of time (10+ seconds) to skip from near the start to near the end of the race, even on a relatively powerful machine. The longer the skip duration and the more cars on track the longer it's going to take. Try skipping through to near the end of a packed 24 hour race and you should be prepared for a long wait.

I realise this doesn't really help the OP, but maybe the explanation will soothe his pain.

edit: I just did a quick test. To skip from the start to the end of a 24 hour race with 27/28 drivers it took me approximately 135 seconds. I'm using a Q9450 and the LFS process was using 25% CPU the entire time, so it pegged one of the quad cores and was entirely CPU limited. This is obviously an extreme example; normal skipping shouldn't take anywhere near as long.

Reading Amp888's post, it seems like the replays requires some processing power.
So it's probably to do with not having enough available resources on your computer hardware.

Make sure you QUIT your internet browser EVERY TIME instead of just closing it.
My Firefox takes 100-500mb RAM and Opera 230-670mb RAM. So I lose 1Gb RAM on programs in the background if I forget to close them down completely.

If it still happens when nothing is running in the background, you could just Format your pc, or just live with the fact your computer can't compute it anymore.(I have no idea what computer hardware you have though)

Maybe Scawen could optimise replay seeking code.

Another thing I want to mention is when you have 30+ GB of replays in your mpr folder, it impossible to even get into replays menu (after few minutes of processing LFS.exe stop responding). And its very slow even with few GB. Probably LFS reads data (lenght, number of players...) from every replay before showing list.