Ditto, tires and tubes. Most effective change I've made, besides better tires, was latex tubes for my wheels with older rims that aren't easily set up for tubeless. Latex tubes aren't nearly as fussy as I'd heard. Just takes a little extra care in rim prep and mounting but they work great.
And bike fit. My lightest road bike is also my slowest because I don't yet have the fit dialed in. I can't do 175 cranks anymore but that Ultegra crankset won't be cheap to replace with the 170 or 172.5 cranks I use on my other bikes. I do just as well on my heavier old steel bike mostly because the fit is perfect.