Don't know what you did, but this is how the 1.15 (and all of its snapshots) run for me: https://imgur.com/a/qhkbdsI

For the record, I can hit 60fps in 1.14.4 almost constantly. In the world I've did this screenshot on, I have no heavy redstone machinery (in fact, none at all as I didn't get to that point yet), just a starter house (slab of dirt with bed, crafting table and furnace inside). I blame Blaze3d.