Hard to pedal

So I’ve bought used Merida Crossway 100.
It’s in very good condition, I have upgraded to it from a modified Gary Fisher (too small frame).

The thing is — compared to the Gary Fisher (which had some drivetrain upgrades), my Merida feels noticeably harder to pedal.
Even though the largest front chainring on the Merida is 48T — which is smaller than what the Gary Fisher had — I feel like I have to pedal harder to reach the same speeds. I get tired faster, and it feels like I’m fighting the bike a bit.

It's not a huge issue on its own, but since I can directly compare it to my Gary Fisher, the difference is very noticeable.

The wheels roll freely.

There’s no external drag slowing the bike down.

Coasting distance is about the same as the Gary Fisher.

So I suspect the issue might be in the bottom bracket.

According to what I found online, my Merida currently has an FSA TH‑7420ST square-taper bottom bracket.

My question is – how much smoother or easier would the ride feel if I replaced it with one of these options?

1. Shimano UN300

2. Shimano UN55

3. Shimano Hollowtech II (e.g., SM-BBR60 + crankset)

I’d prefer to avoid Hollowtech II for now, since it’s more expensive and requires replacing the crankset as well. I’ve also heard that when changing front chainrings, it’s best to replace the whole drivetrain — which I’m not ready to do. Still, I’m curious how much better it would feel in terms of pedaling efficiency and comfort.

And another question:

Can I swap the bottom bracket from my Gary Fisher over to the Merida?

Or if not the bottom bracket, then at least the pedals?

The pedals on the Gary Fisher spin almost effortlessly (silver ones),

While the Merida’s pedals (black ones) barely spin — they stop at about half a turn.