i installed the preview-generator-program from the “store” and (re-)generate all previews with a nightly cronjob. i have ~100GB of pictures and the cronjob takes ~150mins. the previews take up ~83GB (!!!) in /data/appdata_ocjecx05wt96/preview but the gallery really is amazingly fast with it (before that, it wasn’t useable at all.)
so if you have the diskspace and can spare some computing cycles, i’d say it’s really worth a try.
GOOD LUCK!