I ride the Trek Verve 2 (2020). Currently 2,200 miles ridden. It is a heavier sluggish bike compared to the FX series.
I've made a few mods to my Bike:
Shimano Speed Clips
35c tires
Bontrager Ergo Grips (These are a must have on any Flat bar bike)
Bontrager Bar Ends
One thing I've noticed with the Verve, the lazy geometry really requires you to dial in your seat height. Mine is set a little higher than you'd set on a roadie.
For 10 to 20 mile rides my pace is above 16 mph. I hit the wall you're describing many months ago before I started making changes to counteract the heaviness and sluggishness.
When purchasing a bike last year and I was torn between the Verve and FX. I went with the Verve because of the adjustable steam. I have a short clubbed hand, so I was sure if I could handle the more aggressive reach on the FX.
After 2 months of owning the bike I was confident and able to ride in the most forward aggressive position. I'm currently looking at the new Trek FX Sport Carbon 4 as my next bike.
I hope I've listed some helpful suggestions. Happy Riding!