Why would my Mac be slow after inactivity and then get to full speed after 20-30 minutes of use?
The clearest way I can see to see whether data is being paged in or out of RAM is the command line program top. Open a Terminal and run top.

Within the first few lines of output look for  "swapins" and "swapouts". The values at the end tell you how many pages have been swapped in and out of memory. The values in the brackets are the delta for that update (top updates once a second by default).

If your Macbook is swapping Eclipse or your VMs back into RAM when you wake it or come back to it after leaving it for a while, then the swains number should be changing significantly each second.