In an unintended order of influence on my speed
Previously, when I cycled solely on the road, the condition of the earth was not a concern to me and therefore I didn't know how much of an influence it would have on my journey time: Poor roads are poor roads whatever the time of year or whatever the weather. A colleague did tell me the cycle track could get bad over winter, but come autumn I was just a bit more mud splattered, the ground was still pretty firm underneath the film of mud and I thought, "Nah, it'll be fine". A couple of months later though I discovered that stretches of it developed the consistency of wet clay (treacle) and it was VERY hard going on my 42/19 gearing; I need to re-think gearing for next year.
The wind has always got me. Even when I first started with this silly commute shenanigans I had a spell of going "oh this is actually ok on fixed gear", before being hit by the headwinds and going "Oh, no, I really need gears". Had some pretty epic winds this winter.
Food continues to be a major, major issue. In a way I'd kind of got used to not eating enough and so didn't really realise how much a difference food made to my ability to put some effort in; although, calories in does affect calories out (and I need about 4000-4500 a day). Then I ate well. And I woke up and wasn't starving and found I actually had energy to push, hard. And at work I had enough food to eat and not be starving. And so on the way back I found I actually had the energy to push, hard. I can't control the wind or the state of the track and I accept I can't afford a decent bike, but I struggle to accept the lack of food.
Lastly, as much as I wish it was otherwise, the mechanical state of my bicycle probably has the least influence on my speed. The one good thing about fixed gear bikes is you can ride them into an absolutely appalling state of wear and they still... go. I'm only psychologically speed limited in that I don't trust the current state of my bike to push it too hard. And although I can't afford a decent bike I can justify one: In fact, from a return on investment point of view, I could buy a (decent) new bike every single month.