Performance impact of high LS

I increased the LS from 120,000 to around 3 million (according to calculations in the manual, around 16 million would have been ideal, but I wanted to size it such that it would fit within the LWP (Work Pool) as a trial increase first). This seems to have caused a huge performance problem with S1/4 commands - probably because it took more Work Pool than I had anticipated. I then resized it to 1 million online, and refreshed the work pool high water mark, and could see that it was now using only 70% of LWP. Performance improved, but it still not as good as it had been with an LS of 120,000.

I can understand a performance impact when LWP was not sufficient, but could an increase in LS from 120,000 to 1,000,000 cause a performance degradation even if there is plenty of LWP available?