Oh? I have all my PIII's with the POS2009 updates. What did you use to gauge performance loss?
On my dual PIII-850, I noticed that the system felt like it responded more slowly after SP3, but haven't really played around with that system in an everyday setting since about 2012, so cannot say what the updates from 2012 onward have done to its responsiveness.
My dual PIII-S Tualatin's all have updates installed until August 2018 and I didn't notice anything glaring upon reboot. Can you give me some idea as to what you are using to gauge the slowdowns?