[Maxx134]

th problem lies neither with stock or custom rom.
Low mem on either will cause lag.

Older stock 6.1 rom has more mem than stock 6.5rom

The problem is what version apps like Opera U R using.
Newer versions are mem hog.

Manila 2.5 is the biggest mem hog and the main problem.
Some manila versions are compressed and some have better grafics which loead faster.

You have to flash and try.
try a custom rom with latest build like 21905-21907 which handles low mem better over time.
try a custom or stock with manila 2.1 which is faster.
try try try flashing a new rom.
If your not ready to flash, at least a backup and hard reset can remedy an unknown problem causing the lag.