Try installing and using "cpulimit". I think the detrimental effect that such processes (convert/imagemagick batches) have is more likely due to CPU load than I/O, at least that's my impression. I'm not sure about RAM; I actually I'm not sure that convert will load all into RAM at once. For example, even if you convert a PDF with imagemagick, it will first use gs and extract page by page, and then convert the PGN pages on /tmp to whatever specification and format you gave on imagimagick/convert (but in the other hand that's the "other way around", extracting images from PDF rather than generating the PDF, so I don't know).
I have the impression that cpulimit is more effective on longstanding processes or "endogenous" batch processes of a given process (like whatever imagemagick does), but that it doesn't work as well dealing with something that's repeatedly called in a bash loop, for example (unless perhaps it's on a script and cpulimit is issue to restrict the CPU usage of that script). To reduce the CPU load of repeatedly running processes I think the best way is to add some arbitrary delay (sleep) within the loop (it can be somehow adaptive, like modulating the sleep time by some number you get from something like "top" or perhaps preferably some other, less cpu-intensive itself (something on /proc/stat ?), indication of CPU usage).