There are more textures and meshes in the game....what this means is that more memory is consumed by the game...the additional effects themselves though are not what is causing more lag...
The AI is better in some regards (though mostly due to new types of ways it can cheat)...for example, high level AIs get techs randomly for free, even if they don't have the labs...this helps them have all the ship prototypes without a problem...I would expect that the devs will improve the AI once bugs and balance are fixed...
Rebel and Loyalist are separate factions...so, there are 6 factions (of which you choose one to play as from the start)....both TEC factions, for example, are very similiar...but, there are a few key differences in their tech trees, and both have different titans...
The new capital ships are pretty good, particularly the new TEC and Advent caps...the TEC Corsev can capture ships (requires a lot of micro) and has a decent AoE ability (good for dealing with corvettes)...the Advent Discordia has an AoE armor debuff and an AoE that affects ability usage, and it is one of the best caps in the game...the new Vasari ship can spawn different types of temporary frigates that do various things (raw DPS, debuffs, etc)...
New victory conditions include: Capitals (you lose when you lose your capital planet, and you cannot relocate it), Flagship (you start with a beastly ship...if you lose it, you lose), Research (you have to have so many techs, then you research the victory tech), and Occupation (basically a King of the Hill, you rush the occupation planet then hold it for a certain time period)...
The game is 100% standalone...there is no "Diplomacy" or "Entrenchment" mode...it has all the features from previous installments and is simply "Rebellion"...one game...
Corvettes are technically frigates...they are built by frigate factories and abilities targetting frigates will also affect Corvettes...however, they are smaller/cheaper than other combat ships, they use the fighter attack style (that which strikecraft use), and they can shoot in all directions...they are good against pretty much everything, and in mass can work to debuff high priority targets with their abilities...
I highly recommend this game...even if you don't end up being happy with all elements or the game balance, it is still lots of fun, has a great engine, and is easily moddable (there will be lots of mods to try out, I assure you)...