Does merging packages make Sims run better?

No, merging doesn't help. Do check for duplicates and to remove any special characters from the file names. If you don't regularly access your Library area, you could move the Tray folder somewhere outside the Sims 4 folder (sometimes that helps). Higher-poly CC will slow the game more than lower-poly CC. As @Justin says, just having a lot of CC generally or more packs will slow the game depending on your system. Other things that slow it down: playing on bigger lots, playing on more stuffed-with-objects lots, and having more Sims with more relationships.