Opening Writer formulas for editing takes about 4 seconds in 5.1.1.3 on Windows 64 bit and is also slower than with 4.3.7.2.
The original document was authored with Writer 4.3.7.1. After editing, saving, and closing the document several times with Writer 5.1.1.3, performance of the formula editor seems to have improved. During the initial editing cycles there was quite a bit of disk access. Maybe there was some background processing in Writer executing some actions needed to handle the file from a previous version?
Ticket can be closed as far as I'm concerned.