Are there lots of differences from the original game that make this remake slower to speedrun?

Some glitches are different and it has the extra chapters that Gears of War PC has.

I should check notifications more lmao. Slevanas answered the main reason that there are just extra chapters in the campaign.

Other differences that exist but don't affect the time that much: Gears 1:

• More consistent Kim warp
• Seeder 2 skip
• Act 2 fight in the courtyard with a Boomer and some Snipers is often terrifying
• Slightly better Dom RNG in Act 2
• Before enemy troika you can run past an e-hole through a no-light zone to skip the fight
• You can run to the door behind the troika turret without shooting gas cans
• Act 3 Skip from start of underground to Corpser
• Able to take less damage before dying (hardest Gears game for not dying in any%)
• More RNG shotgun
• Clunkier movement and physics
• Double/Triple meleeing exists usually to a player disadvantage
• Flurby himself hasn't made guides for anything

Gears UE:

• Can't do Seeder 2 skip
• Act 2 fight in the first courtyard with a Boomer and some Snipers is wayyy easier
• Very risky to run to the door behind the troika without shooting gas can behind the dumpster
• No-light zone by the e-hole before the troika is blocked off. You have to go around and (probably) fight through it.
• Act 3 skip is only from after the Wretches to the Corpser (maybe 2 minutes slower)
• Act 5 introduces a bunch of previously PC-only chapters, but there are two decent skips in those parts
• Multiple sensitivities for look/aim/zoom makes shooting feel much better
• Movement is much nicer
• Grenades are less stupid and you're able to swag on RAAM much more easily
• Flurby himself has made guides for the skips

That's what I got off the top of my head :)