I'd love to say more but I'm not sure where to start.

Your experience of the game is going to change dramatically depending on when you first played it and how much experience you have with the genre in general. For all my gushing adoration for the game, it is well over a quarter-century old at this point and even then it assumed a fair amount of knowledge and understanding about how the genre works.

When you say "close to beating it" I assume you're talking about the single player campaign, as opposed to the multiplayer death matches?

It's impossible for me to separate my current opinion from the decades of memories I have bound up in the game, but I think the campaign still holds up today, especially in the remastered version. It tells an interesting story with missions that are still fun to play, assuming you enjoy the basic rts build base make units attack enemy.

Like I said, it's hard to tell exactly what part of it you're having problems with, the fundamental gameplay loop is start with a small base, build workers, gather more resources, build bigger base, use bigger base to build army, use army to secure more resource areas to build even bigger base, use biggerer base to build biggest army, win map.

If you're having trouble actually beating the campaigns, 90% of the time the problem is people aren't building enough workers, the exact math gets a little complicated but as a very general rule of thumb you should probably have 25-26 workers for every set of minerals you have a base at, and you should be prioritizing building these workers above basically everything else.

Once you reach something like 8 workers mining minerals you can afford to both constantly build workers and build other units, then once you hit 26 or so you can stop building workers and focus on maxing out your army. Generally speaking the campaign AI is pretty forgiving, concentrating on defense for the first 10 minutes and then pushing out with a maxed out army works on like 90% of the maps.

Beyond that, I don't know what to say about clunky and slow. If the game is too slow, you can actually change what speed it plays at in the options menu, it actually defaults to a medium speed that nobody actually plays the game at, everyone plays at fastest setting, so dunno, maybe that's it. Other than that, units do exactly what you tell them to, no more, no less, that's the nature of this type of game, you're expected to be controlling essentially every unit on the map at all times.