ThinkorSwim ? (
https://www.thinkorswim.com/tos/displayPage.tos?webpage=clientApplication&displayFormat=hide )
By itself probably not much but 4+ monitors suggests running multiple applications at the same time in addition to a chart program.
How old? Like a 2010-11 or 2008-9? No SSD for the OS/Apps ? Maxed out RAM ? A 2011 model with SSD for OS/Apps, separate HDD for 'data' and max RAM probably isn't bad if not doing much heavy duty local trend analyses.
you can launch Activity Monitor to see if Memory , network , or Disk I/O seems bottlenecked.
It certainly is more viable to run 4 monitors on a Mac Pro. That is different from slowness.
Mac Pro might give some upside in being able to run Windows via Virtual Machine for those analytic software titles that don't make it to OS X. The extra RAM and storage I/O help make a VM a little to no compromise solution. Also may help to assign one of the Ethernet ports to that VM (e.g., it is steadily streaming data). That may help with latency on the other port (presuming your router doesn't bottleneck you).
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The older mini isn't so capable. Numbers for various models in answer 14 here:
http://support.apple.com/kb/HT5219?viewlocale=en_US#dispnum
Answer 15 outlines that is really only one if have to switch to BootCamp Windows.
A Mac Pro with more standard video connections won't be as limited if need to boot "raw" into Windows for some reason.