This is a well known problem with PDF.
If you create pdfs with fonts that are not included in Sony Reader (embeded fonts?), then it will be very slow. The same will happen to LRF btw., if you create LRF books in Bulgarian.
There are two ways around it.
1. Add cyrillic fonts to sony reader as it is described on
Russian ebook forum
or
2. you can try this service if it works:
fb2pdf if the source of your books is fb2. This service will convert fb2 to pdf ready for Sony Reader. You will have no problems reading pdfs created on this site.
Although way no. 1 is much better. There was an English speaking member on this forum who has translated Russian instructions to English, because he wanted to prepare his second sony reader for his Russian gf.
Hope it helps.