Why does my bike feel slow, hard to ride and wobblier now? I have a mountain bike with road tires, which I've use for commuting almost every day for over a year now but had barely used for the many years before. It has recently (~1.5 months ago) been serviced, which involved the brakes being adjusted and something being replaced due to damage which I believe was the headset, but I can't be sure because I just agreed to replace what was necessary and the conversation was not in my native language.

Before the service my bike was always a bit wobbly, but I always thought it was normal. Afterwards I realised why other people have to make so much less effort to balance when making hand signals.

At some point after the service my front tire seemed to be going down very quickly, but after checking for punctures and finding nothing I put the tube back in and the tire has held pressure well since. The problem is, since then I feel like I am constantly cycling with a flat despite the pressure being right, and my bike seems wobblier than ever before. I don't know whether I did something wrong when I checked the tire (never done it before) or whether it is something completely different.

Can anyone help?
If your headset was replaced due to crash damaged, well that's a pretty intense crash. I'm going to assume they adjusted it. They also adjusted your brakes. My guess is your problem is two fold:

1) The headset is adjusted too tight, making the bike feel like you have too put too much input into it to turn, and when it does it happens all at once. That would give a wobbly feeling.

2) Check to make sure your brakes are not touching the rim, nor are too close to the rim (less than 3 mm from rim to braking surface). The drag will make you feel like you are constantly pedaling harder.

Good Luck!