Field Upgrade is one of the new features in Call of Duty: Modern Warfare, but it seems players have the wrong idea about its upgraded Pro form.
After making some progress in Call of Duty: Modern Warfare multiplayer, all players unlock a Pro version of Field Upgrade. Pro lets you take two Field Upgrades into a match, rather than one.
Field Upgrades are bonuses and limited-use items that can help you get a leg up. They charge over time, and you can accelerate this even further by getting kills and playing the objective.
As you probably noticed, the different Field Upgrades are split into three categories based on the time they to charge and be usable. There are fast, medium and slow-charging Field Upgrades.
As YouTuber TheXclusiveAce recently discovered, however, the Pro description can be a bit misleading. While you do indeed get to choose between two Field Upgrades when Pro is enabled, the time it takes you to use either one gets completely changed.
Because you get to choose between them on the fly, both of them will charge at the slowest charging rate (of the two you picked). As a result, picking a fast-charging Field Upgrade and a slow-charging one is pointless. No matter how quickly the first one is supposed to charge, the game will always pick the slowest time of the two.
This is why players feel taking Pro slows down the recharge time of their Field Upgrades. The solution is, of course, to not use Pro and just run one Field Upgrade. Although that's going to limit your options, it can be better than waiting a long time for a supposedly fast-charging Field Upgrade.
You can also experiment with using two fast-charging upgrades, since at that point they're both quick enough anyway you won't notice the difference.