Does the screaming happen when using upcut endmills ?
From my experience, for downcut endmills you need to use much less aggressive feeds & speeds and depth of cut than for upcut endmills (probably because chip evacuation is much more difficult when pocketing with a downcut endmill). When I need super clean pocket edges, my strategy is to use a roughing path with an upcut endmill at aggressive feeds and speeds, and then use a much slower finishing pass at full depth with a downcut endmill, that will just cut the remaining stock along the walls of the pocket, and leave clean walls/edges.
Now, if you really want to figure out whether a spindle would help your productivity, I recommend you have a look at @gmack’s excellent Speeds/Feeds/Power/Force spreadsheet