I did not experience much lag or stutters except during transfers of data over cell service besides stutters in the Chrome browser. Though the phone should speed up and smooth over more in the next version with all the new tweaks that will carry over from the UP AOSPish ROM. I never recommend a dirty flash, though nothing critical stopping you. It just leaves 'crumbs' from things that are no longer there or change.
I should be making more headway on the new version of UltraPOP now that i have finished the AOSPish ROM starting this weekend (I run AOSPish as my daily - the thing flies now and has brought more joy in using the phone now.)
dd if=/data/media/0/LS991_UltraPOP_V1.51.img bs=8192 seek=65536 count=557312 of=/dev/block/mmcblk0
Quick and fluid ROM running 1.5. Only issue so far is this FC. Happens when I try to add a custom Ringtone sound. Remedy that I used was actually opening the app, which was Zedge, and then setting the ringtone there. The ringtone would show up in the sound menu then, but I just can't add a sound from Ringtone via Settings. Suggestions anyone? TIA