Ditto, tires. And bar wrap. Made a huge difference in my appreciation of my '89 Ironman.
The bike came with a new set of Vittoria Zaffiro wire bead tires, which were just mediocre. The ride was reasonably fast but harsh at maximum pressure but sluggish and mushy feeling at a comfortable pressure. They skidded much too easily and felt skittish on fast curves. The last straw was how easily the tread cut from ordinary road debris.
After riding those tires for a few months I noticed Jenson selling unpackaged Schwalbe One V-Guards for only $15 each. Presumably an unsold bulk package, possibly intended for team training tires. I bought a pair and in retrospect wish I'd bought out the entire stock of that tire from Jenson. They were that good. That particular Schwalbe was still a bit fragile and got lots of little nicks and cuts, but never bad enough to penetrate the thin puncture shield. The ride was fantastic. Good enough for entry level racing and TT.
Those Schwalbes lasted not quite a year, around 2,500 miles, before I had to replace them due to lots of nicks and cuts. I got some Continental Ultra Sport II in 700x23 and 700x25. Cheap and really good, at any reasonable pressure range. Occasionally I try other tires (including the great looking handmade Conti Grand Prix Classic skinwalls that cost twice as much but aren't twice as good) but I usually go back to the Conti Ultra Sport II because they're outstanding values for about $15 each. I'm planning to try the new Ultra Sport III soon.
Back in 2017 when I bought the bike I was still recovering from serious back and neck injuries and the original bar tape was unbearable. It was typical bar tape, about the thickness of duct tape. I needed a lot of core work, stretching, physical therapy, the works. Meanwhile I improvised with foam rubber padding, held in place with an old inner tube, cut into strips, and stretchy cloth ace wraps for knees, etc. I used that for a few months. Eventually I switched to conventional semi-padded "cork" bar wrap (Arundel, I think?) and have used that for the past couple of years.
I'm not sure any of that stuff made me faster, but it sure made the bike more comfortable so I was "faster" over longer distances because I was comfortable.
But I won't disregard the effect of physical therapy and improving the engine. That made as much difference as the tires and bar wrap.