I'm a commuter cyclist who didn't for the last 18 months really do anything to take care of his bike - though it was stored inside between rides. Took it to the shop 12 months in for a service, but otherwise just pumped up the tyres periodically. This was not very clever, I appreciate.
Having decided to start taking my bike a bit more seriously I decided to start by replacing the chain. My old chain was rusted all over and made a slight grinding sound as I pedalled, so I imagined it was slowing me down by not transferring my pedalling to the wheels as well as it could.
Over the weekend I (eventually) managed to replace the chain with a new one, thanks to Youtube. The new chain is exactly as many links as the old chain: I counted.
But now I could swear I am cycling slower, and my legs hurting more at the same speeds, than before. I'm also commuting to work a few minutes slower than last week. My bike isn't skipping gears, but it is making a noticeable rattling or sliding sound as I pedal (and not when I freewheel down hills). It also occasionally slips, though not much.
Have I somehow made my transmission less efficient? Or am I just going crazy? And if it's the former, does anyone have an idea for what the problem might be? Some Googling has suggested the cassette might need replacing too, but the fact I am changing gears fine makes me think that isn't the case.
Many thanks in advance, Q
EDIT: While trying to implement the suggestions below I found the real problem, which would have been obvious if I'd uploaded photos. I'd threaded the chain wrongly through the deraileur. See attached photo, with the blue line showing the path the chain used to take (I've since rethreaded it properly).
My chain is still slipping under power, which I suspect is the worn cassette. I'll replace that this weekend. But the bike feels much better now and my time into work this morning (with not great traffic conditions) was 20% faster than Mon-Wed.