it seems like you are encountering xruns during recording. Yes you are absolutely correct in assuming that system specs play a part in this, a faster cpu and more RAM would help (in a perfect world)... but if you that's not possible in JACK you can click on the setup button and in the setup dialog box you should be able to find two settings called Frames/Period and secondly Periods/Buffer.
The idea here is to get Frames/Period to be as high as possible without experiencing xruns and to keep Periods/Buffer to the lowest number without experiencing xruns.
In Jack in JACK if you click on thee messages button while JACK is active JACK will print something like this
when an xrun occurs. If this happens start by increasing the Periods/Buffer setting by 1 unit. Then try record or playback again of whatever was causing the xrun. If you still get xruns try dropping the Frames/Period setting, then testing again... you get the point
**** alsa_pcm:xrun of at least 0.017 msecs
on my system that is a intel coreII duo with 2 GB RAM my Frames/Period are set to 512 and Periods/Buffer is set to 2. This allows me to have multiple tracks playing in ardour, with multiple tracks in rosegarden including external synths such as zynaddsubfx playing back without any xruns.
Also please note that when you modify these settings you compromise on Realtime interaction and increase the risk of noticable Latency. Keep an eye on the bottom right hand section of the JACK setup dialog, it estimates what the Latency on your system currently might be based on the settings mentioned above and yes, taking into consideration your system specs