Here's an explanation of the progression. It's a rather complicated system:
There are 3 types of currency: credits, crystals, and crafting parts.
Credits are earned by completing challenges (such as play for 10 minutes as a specific class, get X kills, etc) or at the end of a match. The end of the match bonus is solely based on how long the match is the best I can tell, with no bonus for winning. A galactic conquest match (about 15 minutes) will yield around 300 credits.
Crystals are mostly bought for real money (now disabled), but there are some challenges that reward some.
Crafting parts come from crates and are used to craft star cards (more on those in a second) that either you don't have, or are upgrading to. I think they also come from getting duplicate cards, I can't remember.
Crates contain a random number of star cards and can be purchased with either credits or crystals. A trooper crate contains 4-5 cards and costs 4000 credits. These cards are either white-green-blue qualities (purple is the highest, but can only be crafted). Mostly they are white cards. I've never actually purchased a hero crate (which contains hero star cards, not heroes themselves) or a ship crate, but they're cheaper than the trooper ones. I tend to play mostly Galactic Conquest and I'm not very good, so I'm rarely ever a hero.
There are a number of guns that can only be unlocked by playing that specific class. For example, to unlock the second gun for the trooper, you need 50 kills as a trooper. These guns are not necessarily upgrades, but have different characteristics. Each unlocked gun also has 3 upgrades to unlock (such as scopes) that can only be unlocked with X kills using that specific gun.
Player rank is account wide and is simply "you need X points earned until rank Y". Your rank can never go down, so the better you are, the faster your rank will increase. Player rank is one of the walls to upgrading Star Cards (more on that below).
There are 4 basic classes (trooper, heavy, officer, specialist). 2 upgrade classes (jetpack trooper, and I can't remember the name of the other one). And each hero is their own class essentially. Every vehicle type is it's own class as well. Each class has it's own class level and star card set.
Only the 4 basic classes are available all the time. You have to earn points mid-game to select the other ones. This is a one-time use until you die sort of thing, and some heroes are locked until you unlock them first with credits.
Ok Star Cards. These are upgrades to your abilities on a specific class/hero/ship. For example, on my trooper, I use a star card that increases the explosion radius of my grenades. These bonuses can be quite significant, but don't make you invincible. You never get to the point where a maxed player can taunt you with emotes while you empty clips into them. Star cards ONLY come from crates. There are a few crates you get from rewards, but mostly they're purchased from credits. They are NOT one time use, and any duplicates you might receive get converted into credits.
Every class (basic, upgraded, hero, etc) can equip up to 3 star cards as upgrades, but the slots for card 2 and card 3 are locked until the class reaches a certain class level.
Here is the kicker, the class level is determined by ONLY your collection of star cards for that class, so more/better star cards = higher level.
To upgrade a star card, there are 3 walls.
Your player rank - a function of time.
Class rank - based on your star card collection, which could at one point be bought with real money
Crafting Parts - these come from crates, which could at one point be bought with real money
The highest tier of upgrade (purple) can only be crafted, so they will always require time to be put in to reach the appropriate player rank. But you still have to contend with walls 2 and 3. Note: these restrictions only apply to crafting cards. If you received blue cards in a crate, you can equip them immediately.
You can also purchase white cards with crafting parts. This helps you unlock your 3 star card slots per class. It's fairly easy to get to this point. I don't remember how many white cards were needed to unlock all 3 slots, but I know it was more than 3.
So if you can kind of see there, there is a path for you can spend cash to leap frog forward. I don't feel like it's night and day difference though. Maybe my tune will change when the game starts seeing more maxed out classes, but sometimes people are just significantly better than me so its hard to tell.
I've been playing for maybe a couple of hours every day or so and I'm at rank 14. At rank 15 you're able to upgrade from green to blue. I've only unlocked 1 hero (Iden, the cheapest), so I've spent most of my credits on crates. To this point, based on crafting parts and random cards received, I have been able to keep 1 (and almost a second) class current to my level. I do not have the resources to upgrade other classes, so I mostly stick to the two I have. I am enjoying the game just for the record.