Older (1.1) Mac Pro runs SLOWER with RAM upgrade.

I have an older Mac Pro. I've been running it with 8 megs of RAM until just recently. RAM boards included original 4x512K chips that came with machine, plus 1Mb chips I had added, so I was using all 8 slots to get only 8MB of RAM. Bought 4x2mb to put in slots previously taken with the 512 chips, so new total would be 14Mb.

With new RAM installed (14MB total) the machine ran slower. One thing I noticed is that when the full 14mbs were installed, even though the systems saw them just fine, the additional memory was not being used, according to Activity Monitor - apps like Lightroom and Photoshop were absolutely crawling, but LOTS of free ram showing on the monitor.

All else being equal - plenty of HD space, etc- removing the additional ram restored its previous performance. Now, instead of both RAM boards, I'm just using one memory board with the newer 4x2 Mb modules, totaling 8 mb RAM, while the second board holding older chips worth 6MB sits on my desk. Performance is better but gosh darn it I'd really like to use, and benefit from, the additional 6mb of RAM!

What the heck?