I will have to agree that the loading times alone are what make GTAO horrible to play. Only a few of my friends and I have the patience to wait and get a heist or good session going. We usually get a group together and get into a friends only session and then go to different game modes from there....especially now with the new Finance and Felony update! I'll be damned if I'm gonna have to pay for my crates only to have them blown up by morons.
It is definitely better to have a few loyal friends or friends of friends to play heists together....I never do a heist with randoms.
There are several ways to level up without it feeling so much like monotonous grinding: I love some of the adversaries and those silly player generated LTS's like Rockets and Insurgents, Karumas and mini-guns, Demo derbys....there's too many to list here, but anyway, they are so much fun you don't feel like it's grinding and you get loads of RP and $$$. I have a whole bunch of them bookmarked.
To unlock stuff doing races, you need to do a very short race (Criminal Records), set it on 1 lap, choose super cars only, and have at least one friend who will let you win. Same with an air race, rally race and a sea race to unlock some paint colors.
Chalk, send me a message if you want to play sometime...or anyone else on Xbox (send friend request if you're not on my list)