Why does opening the Tools menu take so long?
It's an RXTX issue. RXTX is the native serial library used to make the IDE talk to the boards.

On some Windows installation, with some Bluetooth things (both dongles or integrated), when enumerating serial devices, these dongles scan the air for remote serial devices. Hence, the IDE stays stuck, waiting for RXTX serial device enumeration to end. This issue has been acknowledged a lot of times, see for example #1375, #532, #200 and #143

As far as I understand, this issue does not affect linux or mac because of the different way the scan occurs. On linux it's a plain filtered file listing of /dev. Similarly it's on mac. On windows, it uses native API calls.

Some fixes are available in the 1.5 series but I suggest switching to the latest 1.5.6 release as RXTX was replaced by JSSC, a more modern and simpler library.