Trying to shed some light on the "too long and boring" issue from perhaps a slightly different angle: I think what contributes to this feeling is a combination of "player expertise" and "lack of escalation".
Players that qualify for Arbitrations are likely to be veterans. The're already very used to the game modes offered by Arbitrations, and the one new element they bring to the table - a lack of revives - can be largely countered with meta picks and cheesy builds, meaning the actual gameplay either stays the same as per usual or even becomes more boring as players "optimize the fun away" with the builds that the high risk system encourages you to use. Arbitrations do offer some power boni for picking off-color frames and weapons, but first and foremost Arbitrations are high challenge content, which means anyone who takes the challenge as seriously as it presents itself will bring safe picks.
Lack of escalation is something that affects both players that bring optimal cheese and those that handicap themselves for fun. Both types will still want to bring a certain minimum level of power since there's no fun in dying immediately. Usually this means that players will at least bring enough power to last to RotC for its exclusive rewards, tho Arbitrations encourage you to bring even more firepower so you can reach multiple RotCs. So until you hit the point where your loadout starts to struggle - RotC or later - the gameplay is going to stay at a relatively easy level. Arbitrations are supposed to be about challenge, but there's a big windup time until you actually get to that challenge, which is why a big chunk of the gameplay can feel like you're performing monotonous tasks in a "waiting room" until you get to the part where Arbitrations start to feel as intended.
Both issues also work together in the case of natural escalation - the game spawning heavier enemy types more frequently over time - because veterans are already very used to the escalated state of missions and in fact build around being able to trivialize even the heavy enemy types, that become virtually indistinguishable from cannonfodder if they die just as quickly as grunts before being able to get a shot off. So the game's own attempt at escalation falls flat.
Basically, I think players would be more okay with slower reward payouts if Arbitrations felt like its own beast. 10 minutes only feel like a long time if you're unengaged and not having fun.
Currently it mostly feels like normal gameplay with more stretched-out rewards, which obviously feels like a downgrade compared to normal gameplay with normal-frequency rewards. In fact, they kinda remind me of old Void missions or those 15min lvl10 GotL Survivals. It's more punishing if you fall asleep at the wheel and get OHK'd because of a stupid mistake, but overall my gameplay style hasn't been affected by that fact.