What is your bike goal for this bike?

It seems you are disappointed in a perceived speed difference between your two current bikes (perfectly fair enough). They both seem to be touring bikes, however, and you are contemplating other touring bikes to replace one of them.

If you are always touring/always need racks then that makes sense. It's not the type of bike I'd be considering if speed were my issue.

Sounds like you are pretty happy with your 520. So, have you thought about getting a more performance oriented road bike (this doesn't have to mean an aero "crotch rocket"; they make these with more a more relaxed, century oriented, geometry two). Something like maybe a performance setup Madone (would not be my personal first choice, but keeping the Trek theme).

A few mph difference can be anything, even something as simple as tire choice, wheel weight differences could be a factor as well, so could a few mm to cm difference in your position between the two.

I know LHT are a favorite on this forum, but I, personally, would not consider one for anything other than a commuter or loaded tourer and I wouldn't expect it to go fast (but fast, for me, is holding on to a 20mph average pace with plenty of surges).

So it comes down to what you are wanting the bike to do.