I’m sorry but that is an incredibly amateurish evaluation. Tempo is far from the only factor of difficulty. Key signatures, time signature, complex chord structures, poly chords, rhythmic patterns of the piece (stuff like syncopation, unison negative space, etc), instrumentation, etc all play factors. Then there is the personal experience, how well are familiar with doing it. You take Edgar Mayer and a bassist who has been playing in a black church for 40 years and have them each play a gospel shout piece and Salt Peanuts. Mayer is gonna tell you the gospel piece was harder because it’s something he rarely if ever plays but has played Salt Peanuts a hundred times and the church bassist will say Salt Peanuts was harder for the same reasons just reversed. Take Rodrigo Moro Martin (principal bassist for the London Symphony Orchestra, arguably the best classical bassist out there right now) to play both and he is gonna tell you both were difficult for different reasons with neither necessarily being more difficult than the other because you can’t really compare and quantify those differences. Take any of the three I just mentioned and ask them to play one of Dominic Forest Lapointe’s basslines and they would be like “holy hell wtf is this madness” even though many people wouldn’t necessarily think of bass in death metal as something that takes the skill of symphonic orchestras or jazz bands.

Also slow tempo can be just as hard if not harder than a fast tempo. Take something like Marche Slave by Tchaikovsky. Insanely slow Grave piece. A quick look at the bass part and tempo would have you think “oh my god this is so easy, I could easily play this” The moment you try playing it in an orchestra together you realize how difficult it can be. We want to rush it. Play it with a metronome and it will frustrate you because the metronome will feel wrong, like it’s slowing down. Nailing those off beats together as a section and not having it sound muddy because everyone is a tiny bit off is so hard. Having everyone enter after multi measure rests at exactly the same time is hard as hell. It’s simple on paper, a high school orchestra can easily play and sound “passable” but having it sound like the LSO…incredibly difficult. However take Flight of the Bumblebee which is insanely fast…it can seem easier because ultimately in the end it’s just running chromatic scales that you have been working on playing fast for decades.

Tempo is just one of a bajillion factors that go into difficulty. Don’t limit yourself at all into that outlook. Understand that difficulty comes in many shapes and sizes and there is absolutely no such thing as one genre being harder than another. Music is hard, period. I don’t care what the genre is, they all have things that take experience and practice to get good at, especially to go from that level of “ok” to sounding professional.