Assuming optimal conditioning for our age, etc., I've found only a few cost effective bits of gear that made a significant difference:
The most aero position you can hold on any bike. Probably matters more than the frame material, or even aero wheels, etc. Plenty of wind tunnel tests to show this on YouTube. Check Oliver Bridgewood's videos on GCN and his previous channel, especially his attempt at the one hour record. Ollie is the most relatable presenter on GCN because he's not a pro, never been a pro, never could have been a pro, but he's a strong recreational rider and an everyman type of guy.
Tires and tubes. Decent supple tires and latex tubes made a big difference to me in lower rolling resistance and, more important to me, comfort. If I'm comfortable I can hold an aero tuck longer.
Aero kit. A cheap knockoff of an older Giro aero helmet with face shield, and some affordable Pearl Izumi aero jerseys helped a bit as well. Supposedly even aero socks make a significant difference, although I don't know whether those have been compared to shaved bare legs and ankles in a wind tunnel. Full custom aero suit if your budget allows. But a helmet may be the biggest factor in getting aero, after the optimal position on the bike. Makes sense that the biggest body parts facing the wind -- torso and head -- would benefit the most from aero kit and getting the most aero tuck we can manage and hold for awhile.
Getting a position on the bike that I can hold for an hour without excruciating pain. Due to neck injuries and cervical stenosis I can't manage most aero bar positions longer than a few minutes at a time, but even those outdated, uncomfortable old school Scott clip-ons like Greg LeMond used in the late 1980s were good for a 1 mph boost even if I couldn't hold the position longer than a few minutes at a time before sitting up. Carbon fiber Profile aero bars were more comfortable and helped as well, but two years after trying those I can't handle that position for more than a few minutes either. This year I had to unslam my bar and stem and raised the bar to where it's only an inch or so below saddle height. It's much more comfortable and I can hold a reasonably aero tuck with hands on the hoods, forearms nearly parallel with the ground, longer than I could with the bar and stem slammed. So, for me, less aero is better because it works better for me. Maybe after a cervical spine ablation procedure later this year I'll be able to get more aero again, at least until the effect wears off.
Little things like a clean bike and chain, chain lube, etc.
But bike frame material or design? Nah, makes no difference in my rides. I have personal best times on Strava segments ranging from 1-6 miles (I don't pay attention to shorter segments) riding my 1989 Centurion Ironman steel bike, 1993 Trek 5900 OCLV (carbon fiber but old school standard tubing and diamond frame), and 2010 or so Diamondback Podium 5 with internally routed cables. Significant weight differences between 'em, but we have no mountains here. None of our climbs are long enough or consistent enough to matter. It's all rollers with lots of semi-steep but very short sprint-climbs. My best time on a favorite 6 mile time trial roller coaster segment is on my steel bike, from 2017, I think. Just a combination of perfect conditions and physical fitness. I haven't yet matched my 2017 conditioning, after a 2018 bout with thyroid cancer and getting hit by a car.
I've tried semi-aero wheels vs my favorite low profile old school climbing rims. No consistent differences.
I've tried various bar wraps, and no wraps, just bare handlebars. No consistent differences that might be attributable to aerodynamics.