They're not very efficient, but they are the only engines that could do the job. I just ended up slapping a lot of fuel to the first stage. I'll try that. Also:
It affects unfortunate users with low-end computers the most (obviously) but it might ever so slightly decease performance with even the best rigs. Also, looking up at the sky reduces lag too.
Apparently, lag slightly decreases the performance of crafts in KSP. Turning the settings down and putting the max Delta time to 1.00 should yield better results.