When I install Adobe Reader X (v10) or v11, I notice a huge degradation in search speed. I first noticed this last year, and I just re-verified my results with their latest version (v10.1.3). I don't know if this is unique to my system, or if you might have similar issues.
I am using a wee Acer Aspire One netbook. It has an Intel Atom processor and runs Windows XP SP3 at 1.6GHz with 1GB RAM. I have a set of PDF files in which I search for articles. There are 16 files, totalling 600MB, with about 800 pages in each PDF file.
My test was to use Edit->Search to look for a single word across the entire set of files, with no other options specified. Search returned 14 files with 54 instances. I have a few saved timings:
Adobe Reader v9.1.0 24 seconds.
Adobe Reader v9.2.0 15 seconds.
Adobe Reader v9.4.6 12 seconds.
Adobe Reader v9.5.0 14 seconds.
Adobe Reader v9.5.1 15 seconds.
Adobe Reader v9.5.2 14 seconds.
Adobe Reader v10.1.3 841 seconds. (14m 01s)
Adobe Reader v11.0.0 991 seconds. (16m 31s)
Un-installing v10 or v11 and re-installing v9.5 immediately restores the faster performance.